with a quick fix (check null pointer) : 
diff --git a/ext/spl/spl_iterators.c b/ext/spl/spl_iterators.c
index c6181d9..bb86e76 100644
--- a/ext/spl/spl_iterators.c
+++ b/ext/spl/spl_iterators.c
@@ -1682,7 +1682,7 @@ static inline void spl_dual_it_rewind(spl_dual_it_object *intern TSRMLS_DC)
 {
 	spl_dual_it_free(intern TSRMLS_CC);
 	intern->current.pos = 0;
-	if (intern->inner.iterator->funcs->rewind) {
+	if (intern->inner.iterator && intern->inner.iterator->funcs->rewind) {
 		intern->inner.iterator->funcs->rewind(intern->inner.iterator TSRMLS_CC);
 	}
 }

will get:
Fatal error: Uncaught exception 'Exception' with message 'Cannot traverse an already closed generator' in /tmp/1.php:9
Stack trace:
#0 [internal function]: AppendIterator->rewind()
#1 /tmp/1.php(9): iterator_to_array(Object(AppendIterator))
#2 {main}
  thrown in /tmp/1.php on line 9


which seems not quite reasonable error message. 

@nikic, what do you think?

@laruence: There are two bugs here:
a) It does not handle exceptions thrown from get_iterator(). This is what you fixed.
b) It calls get_iterator() multiple times for some reason, which is why there is an exception in the first place.

I experience the same problem with PHP 5.6.19. @nikic are there any chances to fix it?

Same bug on 5.5.9

This crash is present in 5.6.24.

Possibly related:

-------------------------------------------------------
#0  zend_mm_alloc_small (size=56, bin_num=6, heap=0x7f8c9de00040) at /src/php-7.1.2/Zend/zend_alloc.c:1261
#1  _emalloc_56 () at /src/php-7.1.2/Zend/zend_alloc.c:2336
#2  0x000000000080d7a0 in _array_init (arg=arg@entry=0x7ffffdaf06f0, size=size@entry=0) at /src/php-7.1.2/Zend/zend_API.c:1060
#3  0x0000000000824f31 in zend_fetch_debug_backtrace (return_value=return_value@entry=0x7ffffdaf0780, skip_last=skip_last@entry=0, options=options@entry=0, 
    limit=limit@entry=0) at /src/php-7.1.2/Zend/zend_builtin_functions.c:2612
#4  0x000000000082bb0f in zend_default_exception_new_ex (class_type=0x2015d80, skip_top_traces=0) at /src/php-7.1.2/Zend/zend_exceptions.c:216
#5  0x000000000080df3b in _object_and_properties_init (arg=arg@entry=0x7ffffdaf07f0, class_type=class_type@entry=0x2015d80, properties=properties@entry=0x0)
    at /src/php-7.1.2/Zend/zend_API.c:1302
#6  0x000000000080e047 in _object_init_ex (arg=arg@entry=0x7ffffdaf07f0, class_type=class_type@entry=0x2015d80)
    at /src/php-7.1.2/Zend/zend_API.c:1310
#7  0x0000000000438429 in zend_throw_exception (exception_ce=0x2015d80, exception_ce@entry=0x0, message=message@entry=0xdf6b78 "Cannot traverse an already closed generator", 
    code=code@entry=0) at /src/php-7.1.2/Zend/zend_exceptions.c:922
#8  0x00000000008365ee in zend_generator_get_iterator (ce=<optimized out>, object=0x7f8c9de13980, by_ref=<optimized out>)
    at /src/php-7.1.2/Zend/zend_generators.c:1186
#9  0x00000000006e59c5 in spl_iterator_apply (obj=<optimized out>, apply_func=0x6df320 <spl_iterator_to_values_apply>, puser=puser@entry=0x7f8c9de13920)
    at /src/php-7.1.2/ext/spl/spl_iterators.c:3495
#10 0x00000000006e5af5 in zif_iterator_to_array (execute_data=<optimized out>, return_value=0x7f8c9de13920)
    at /src/php-7.1.2/ext/spl/spl_iterators.c:3590
#11 0x00000000008aa572 in ZEND_DO_FCALL_BY_NAME_SPEC_RETVAL_USED_HANDLER () at /src/php-7.1.2/Zend/zend_vm_execute.h:876
#12 0x000000000085215b in execute_ex (ex=<optimized out>) at /src/php-7.1.2/Zend/zend_vm_execute.h:429
#13 0x0000000000836c32 in zend_generator_resume (orig_generator=0x7f8c9de00040) at /src/php-7.1.2/Zend/zend_generators.c:817
#14 0x00000000008696b9 in ZEND_FE_FETCH_R_SPEC_VAR_HANDLER () at /src/php-7.1.2/Zend/zend_vm_execute.h:16816
#15 0x000000000085215b in execute_ex (ex=<optimized out>) at /src/php-7.1.2/Zend/zend_vm_execute.h:429
#16 0x0000000000836c32 in zend_generator_resume (orig_generator=0x7f8c9de00040) at /src/php-7.1.2/Zend/zend_generators.c:817
#17 0x00000000008696b9 in ZEND_FE_FETCH_R_SPEC_VAR_HANDLER () at /src/php-7.1.2/Zend/zend_vm_execute.h:16816
#18 0x000000000085215b in execute_ex (ex=<optimized out>) at /src/php-7.1.2/Zend/zend_vm_execute.h:429
#19 0x00000000008ad410 in zend_execute (op_array=0x7f8c9de7d000, op_array@entry=0x7f8c9d089d00, return_value=return_value@entry=0x7f8c9de13500)
    at /src/php-7.1.2/Zend/zend_vm_execute.h:474
#20 0x000000000080b834 in zend_execute_scripts (type=type@entry=8, retval=0x7f8c9de13500, retval@entry=0x0, file_count=file_count@entry=3)
    at /src/php-7.1.2/Zend/zend.c:1475
#21 0x00000000007abfb0 in php_execute_script (primary_file=0x7ffffdaf3060) at /src/php-7.1.2/main/main.c:2537
#22 0x00000000008af59a in do_cli (argc=-1646264256, argv=0x0) at /src/php-7.1.2/sapi/cli/php_cli.c:993
#23 0x000000000043a36c in main (argc=-1646264256, argv=0x0) at /src/php-7.1.2/sapi/cli/php_cli.c:1381

first rewind is invoked by spl_append_it_next_iterator(), second one is invoked by spl_iterator_apply(). 

so maybe some flag telling that append iterator is already rewinded would solve it?

Same here, PHP 7.2.0, AppendIterator is pretty much unusable because of this behaviour.

You just need to use NoRewindIterator:

<?php
function foo() {
        foreach ([] as $foo) {
                yield $foo;
        }
}
$append = new AppendIterator();
$append->append(new NoRewindIterator(foo()));

var_dump(iterator_to_array($append));

The segfault has been already fixed as bug #72684; the other issue
mentioned by Nikita is tracked as bug #72692, so I'm closing this
ticket as duplicate.