Added three lines to the beginning of sapi_register_post_entry():

printf("SG=%p\n", TSRMG_BULK(sapi_globals_id, sapi_globals_struct*)); // Line 951
printf("SG(sapi_started)=%d\n", (int)SG(sapi_started)); // Line 952
printf("EG=%p\n\n", TSRMG_BULK(executor_globals_id, zend_executor_globals *)); // Line 953

Modified the test script a bit:

int main(int argc, char** argv)
{
        printf("First block\n");
        PHP_EMBED_START_BLOCK(argc, argv)
        PHP_EMBED_END_BLOCK();

        printf("Second block\n");
        PHP_EMBED_START_BLOCK(argc, argv)
        PHP_EMBED_END_BLOCK();
}

Compiled and run:

First block
SG=0x55c1cdb6c190
SG(sapi_started)=0
EG=0x21

SG=0x55c1cdb6c190
SG(sapi_started)=0
EG=0x21

SG=0x55c1cdb6c190
SG(sapi_started)=0
EG=0x55c1cdb6f9b0

SG=0x55c1cdb6c190
SG(sapi_started)=0
EG=0x55c1cdb6f9b0

Second block
SG=0x55c1cdbf8d30
Помилка адресування (збережено знімок оперативної пам’яті)

Looks like this has something to do with SG(sapi_started).

Running under valgrind:

$ USE_ZEND_ALLOC=0 valgrind ./test
==16297== Memcheck, a memory error detector
==16297== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==16297== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==16297== Command: ./test
==16297== 
First block
SG=0x100adf70
SG(sapi_started)=0
==16297== Invalid read of size 8
==16297==    at 0x53462D3: printf (stdio2.h:104)
==16297==    by 0x53462D3: sapi_register_post_entry (SAPI.c:953)
==16297==    by 0x53463E1: sapi_register_post_entries (SAPI.c:940)
==16297==    by 0x534964F: php_setup_sapi_content_types (php_content_types.c:64)
==16297==    by 0x533823B: ts_allocate_id (TSRM.c:259)
==16297==    by 0x5460FF3: php_embed_init (php_embed.c:182)
==16297==    by 0x108989: main (test.c:7)
==16297==  Address 0x100adf18 is 8 bytes before a block of size 8 alloc'd
==16297==    at 0x4C2DA5F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16297==    by 0x4C2FDDF: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16297==    by 0x53381E8: ts_allocate_id (TSRM.c:255)
==16297==    by 0x5460FF3: php_embed_init (php_embed.c:182)
==16297==    by 0x108989: main (test.c:7)
==16297== 
EG=(nil)

SG=0x100adf70
SG(sapi_started)=0
EG=(nil)

SG=0x100adf70
SG(sapi_started)=0
EG=0x100b25f0

SG=0x100adf70
SG(sapi_started)=0
EG=0x100b25f0

Second block
SG=0x112dfce0
==16297== Invalid read of size 8
==16297==    at 0x534629E: sapi_register_post_entry (SAPI.c:952)
==16297==    by 0x53463E1: sapi_register_post_entries (SAPI.c:940)
==16297==    by 0x534964F: php_setup_sapi_content_types (php_content_types.c:64)
==16297==    by 0x533823B: ts_allocate_id (TSRM.c:259)
==16297==    by 0x5460FF3: php_embed_init (php_embed.c:182)
==16297==    by 0x108AA2: main (test.c:11)
==16297==  Address 0x100adec0 is 0 bytes inside a block of size 32 free'd
==16297==    at 0x4C2ED5B: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16297==    by 0x533803F: tsrm_shutdown (TSRM.c:190)
==16297==    by 0x54611C6: php_embed_shutdown (php_embed.c:229)
==16297==    by 0x108A7F: main (test.c:8)
==16297==  Block was alloc'd at
==16297==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16297==    by 0x533830D: allocate_new_resource (TSRM.c:279)
==16297==    by 0x5338570: ts_resource_ex (TSRM.c:368)
==16297==    by 0x5460FCA: php_embed_init (php_embed.c:176)
==16297==    by 0x108989: main (test.c:7)
==16297== 
==16297== Invalid read of size 8
==16297==    at 0x53462A1: sapi_register_post_entry (SAPI.c:952)
==16297==    by 0x53463E1: sapi_register_post_entries (SAPI.c:940)
==16297==    by 0x534964F: php_setup_sapi_content_types (php_content_types.c:64)
==16297==    by 0x533823B: ts_allocate_id (TSRM.c:259)
==16297==    by 0x5460FF3: php_embed_init (php_embed.c:182)
==16297==    by 0x108AA2: main (test.c:11)
==16297==  Address 0x11123650 is 0 bytes inside a block of size 376 free'd
==16297==    at 0x4C2ED5B: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16297==    by 0x5338034: tsrm_shutdown (TSRM.c:189)
==16297==    by 0x54611C6: php_embed_shutdown (php_embed.c:229)
==16297==    by 0x108A7F: main (test.c:8)
==16297==  Block was alloc'd at
==16297==    at 0x4C2FD4F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16297==    by 0x53381E8: ts_allocate_id (TSRM.c:255)
==16297==    by 0x10ECAEF1: ???
==16297==    by 0x53B37FA: zend_startup_module_ex (zend_API.c:1843)
==16297==    by 0x53B5CA5: zend_startup_module (zend_API.c:2454)
==16297==    by 0x53BAC04: zend_extension_startup (zend_extensions.c:184)
==16297==    by 0x539F64F: zend_llist_apply_with_del (zend_llist.c:171)
==16297==    by 0x53BAD26: zend_startup_extensions (zend_extensions.c:205)
==16297==    by 0x533B397: php_module_startup (main.c:2305)
==16297==    by 0x5460F6C: php_embed_startup (php_embed.c:109)
==16297==    by 0x54610C1: php_embed_init (php_embed.c:200)
==16297==    by 0x108989: main (test.c:7)
==16297==
==16297== Invalid read of size 1
==16297==    at 0x53462A6: sapi_register_post_entry (SAPI.c:952)
==16297==    by 0x53463E1: sapi_register_post_entries (SAPI.c:940)
==16297==    by 0x534964F: php_setup_sapi_content_types (php_content_types.c:64)
==16297==    by 0x533823B: ts_allocate_id (TSRM.c:259)
==16297==    by 0x5460FF3: php_embed_init (php_embed.c:182)
==16297==    by 0x108AA2: main (test.c:11)
==16297==  Address 0x1b4 is not stack'd, malloc'd or (recently) free'd
==16297==
==16297==
==16297== Process terminating with default action of signal 11 (SIGSEGV)
==16297==  Access not within mapped region at address 0x1B4
==16297==    at 0x53462A6: sapi_register_post_entry (SAPI.c:952)
==16297==    by 0x53463E1: sapi_register_post_entries (SAPI.c:940)
==16297==    by 0x534964F: php_setup_sapi_content_types (php_content_types.c:64)
==16297==    by 0x533823B: ts_allocate_id (TSRM.c:259)
==16297==    by 0x5460FF3: php_embed_init (php_embed.c:182)
==16297==    by 0x108AA2: main (test.c:11)

The very first error can probably be ignored - it looks like executor globals are not ready yet.

The second one probably means that SAPI globals structure has not been (properly) allocated.

Could be related:

7.0.17-zts-debug works
7.0.18-zts-debug does not work (crashes in php_startup_ticks(), main/php_ticks.c:32)
7.1.0-zts-debug does not work

Disregard the previous comment, 7.0.17-zts-debug was actually NTS :-(

7.0.0-zts-debug crashes as well (malloc.c:3759: _int_malloc: Assertion `(unsigned long) (size) >= (unsigned long) (nb)' failed.).

NTS versions seem to work so the bug seem to affect only ZTS.