Fixed segfault with some compilers in store.c

There was no sequence point, so the compiler might first pass the value of key (which is then ((char *)bogus heap memory)) to zend_hash_del_key_or_index and only then fetch the value of key via zend_hash_get_current_key_ex.
Separating it into two different statements inserts a sequence point there so that key is guaranteed to be a valid pointer.