Address review comment

Author: Girgias

ext/pdo/pdo_stmt.c

@@ -662,8 +662,8 @@ static bool pdo_call_fetch_object_constructor(zend_function *constructor, HashTa
 	return failed;
 }
 
-/* perform a fetch.
- * Stores values into return_value according to HOW. */
+/* Performs a row fetch, the value is stored into return_value according to HOW.
+ * retun_value MUST be safely destroyable as it will be freed if an error occurs. */
 static bool do_fetch(pdo_stmt_t *stmt, zval *return_value, enum pdo_fetch_type how, enum pdo_fetch_orientation ori, zend_long offset, zval *group_key) /* {{{ */
 {
 	int flags;
@@ -2053,6 +2053,7 @@ static void pdo_stmt_iter_dtor(zend_object_iterator *iter)
 
 	if (!Z_ISUNDEF(I->fetch_ahead)) {
 		zval_ptr_dtor(&I->fetch_ahead);
+		ZVAL_UNDEF(&I->fetch_ahead);
 	}
 }
 
@@ -2093,6 +2094,7 @@ static void pdo_stmt_iter_move_forwards(zend_object_iterator *iter)
 
 	if (!Z_ISUNDEF(I->fetch_ahead)) {
 		zval_ptr_dtor(&I->fetch_ahead);
+		ZVAL_UNDEF(&I->fetch_ahead);
 	}
 
 	if (!do_fetch(stmt, &I->fetch_ahead, PDO_FETCH_USE_DEFAULT,