Description:
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Under some circumstences, spl_recursive_it_dtor() crashes because it accessed a NULL pointer free'ed before by spl_RecursiveIteratorIterator_free_storage().

This is related to #51697 , seems very similar. 51697 is marked as closed though

Test script:
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I could not isolate easily the behavior.
But launching Symfony tests with latest PHPUnit under PHP 7.1 can trigger the crash in the Debug component of Symfony.

Stack trace is then
#0  0x00000000008605fe in zval_get_type (pz=0xd0) at /home/julien.pauli/workspace/php/Zend/zend_types.h:332
#1  0x00000000008610ae in spl_recursive_it_dtor (_iter=0x7fffdd787300) at /home/julien.pauli/workspace/php/ext/spl/spl_iterators.c:178
#2  0x0000000000a95a2f in iter_wrapper_free (object=0x7fffdd787300) at /home/julien.pauli/workspace/php/Zend/zend_iterators.c:69
#3  0x0000000000abef48 in zend_objects_store_free_object_storage (objects=0x14bfa78 <executor_globals+824>) at /home/julien.pauli/workspace/php/Zend/zend_objects_API.c:99
#4  0x0000000000a516d5 in shutdown_executor () at /home/julien.pauli/workspace/php/Zend/zend_execute_API.c:363
#5  0x0000000000a6b6ea in zend_deactivate () at /home/julien.pauli/workspace/php/Zend/zend.c:999
#6  0x00000000009d122b in php_request_shutdown (dummy=0x0) at /home/julien.pauli/workspace/php/main/main.c:1877

This happens because spl_RecursiveIteratorIterator_free_storage() has been called, and free'ed object->iterators but did not reset the level (object->level). Then spl_recursive_it_dtor tries to read from object->iterators (NULL).

The patch is simply to reset the level while dtor'ing.

Expected result:
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No crash

Actual result:
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Crash with NULL pointer dereference