Fixed a potential segfault in zend_objects_store_free_object_storage()

Under the special circumstance where a garbage collected objects bucket slot was not reused until the end of the script, we get access into freed memory...
No test added as it usually is valgrind-only, and only sometimes when the memory happens to have changed (i.e. (GC_FLAGS(obj) & IS_OBJ_FREE_CALLED) == 0), it actually *may* segfault

NEWS

@@ -20,6 +20,8 @@ PHP                                                                        NEWS
     phpize). (Yuji Uchiyama)
   . Fixed bug #72641 (phpize (on Windows) ignores PHP_PREFIX).
     (Yuji Uchiyama)
+  . Fixed potential segfault in object storage freeing in shutdown sequence.
+    (Bob)
 
 - COM:
   . Fixed bug #72569 (DOTNET/COM array parameters broke in PHP7). (Anatol)

Zend/zend_gc.c

@@ -1097,6 +1097,7 @@ ZEND_API int zend_gc_collect_cycles(void)
 
 				if (EG(objects_store).object_buckets &&
 				    IS_OBJ_VALID(EG(objects_store).object_buckets[obj->handle])) {
+					EG(objects_store).object_buckets[obj->handle] = SET_OBJ_INVALID(obj);
 					GC_TYPE(obj) = IS_NULL;
 					if (!(GC_FLAGS(obj) & IS_OBJ_FREE_CALLED)) {
 						GC_FLAGS(obj) |= IS_OBJ_FREE_CALLED;