Description:
------------
Since upgrading from PHP 7.0.7 to PHP 7.0.8 (using APCu 5.1.5) I've encountered an issue with using APCu to cache data.

Basically caching the value works correctly, I'm also only noticing this occurring when caching arrays.

For some reason the first call to apcu_fetch() works fine, but on the second call it presents me with a zend_mm_heap corrupted error and absolutely kills the page with no nice 404 or 500.

Rolling back to 7.0.7 fixes the issue so this really seems tied to 7.0.8. I was thinking perhaps it was just an issue with 7.0.8 overall however when I setup the same versions on a cloud 9 instance the error did not reproduce.

Steps to reproduce:

Fresh CentOS 7.2 x64

rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
rpm -Uvh https://mirror.webtatic.com/yum/el7/webtatic-release.rpm
yum install php70w php70w-opcache php70w-fpm 
yum install php70w-pecl-apcu

Run test script (make sure apcu is enabled + if running by command line apcu.enable_cli is true)

Test script:
---------------
<?php
$key = 'cache_key';

apcu_store($key, ['net' => 23]);

$result = apcu_fetch($key);

print_r($result); // At this point result is as expected.

$result = apcu_fetch($key); // This causes error: `zend_mm_heap corrupted`

print_r($result); // Never gets printed

Expected result:
----------------
Expect to not have a zend_mm_heap corrupted and get the stored value.

Actual result:
--------------
#0  zend_mm_free_heap (ptr=0x7fe215542370, heap=0x7fe226600040) at /usr/src/debug/php-7.0.8/Zend/zend_alloc.c:1408
#1  _efree (ptr=0x7fe215542370) at /usr/src/debug/php-7.0.8/Zend/zend_alloc.c:2466
#2  0x00007fe22ae64b19 in zend_string_release (s=<optimized out>) at /usr/src/debug/php-7.0.8/Zend/zend_string.h:271
#3  zend_array_destroy (ht=0x7fe226656268) at /usr/src/debug/php-7.0.8/Zend/zend_hash.c:1341
#4  0x00007fe22ae98c45 in zend_assign_to_variable (value_type=4 '\004', value=0x7fe226613100, variable_ptr=0x7fe2266130a0) at /usr/src/debug/php-7.0.8/Zend/zend_execute.h:103
#5  ZEND_ASSIGN_SPEC_CV_VAR_HANDLER () at /usr/src/debug/php-7.0.8/Zend/zend_vm_execute.h:33580
#6  0x00007fe22ae924eb in execute_ex (ex=<optimized out>) at /usr/src/debug/php-7.0.8/Zend/zend_vm_execute.h:414
#7  0x00007fe22ae43c19 in dtrace_execute_ex (execute_data=<optimized out>) at /usr/src/debug/php-7.0.8/Zend/zend_dtrace.c:83
#8  0x00007fe21fc754df in xdebug_execute_ex () from /usr/lib64/php/modules/xdebug.so
#9  0x00007fe22aedc6df in zend_execute (op_array=op_array@entry=0x7fe22667e000, return_value=return_value@entry=0x0) at /usr/src/debug/php-7.0.8/Zend/zend_vm_execute.h:458
#10 0x00007fe22ae53933 in zend_execute_scripts (type=type@entry=8, retval=retval@entry=0x0, file_count=file_count@entry=3) at /usr/src/debug/php-7.0.8/Zend/zend.c:1427
#11 0x00007fe22adf4b78 in php_execute_script (primary_file=primary_file@entry=0x7ffe7e98b860) at /usr/src/debug/php-7.0.8/main/main.c:2494
#12 0x00007fe22aede348 in do_cli (argc=2, argv=0x7fe22ccd3a90) at /usr/src/debug/php-7.0.8/sapi/cli/php_cli.c:974
#13 0x00007fe22ac9207a in main (argc=2, argv=0x7fe22ccd3a90) at /usr/src/debug/php-7.0.8/sapi/cli/php_cli.c:1344