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`. (Assembly told me that llvm-gcc really did so...)
Separating it into two different statements inserts a sequence point there so that key is guaranteed to be a valid pointer.