box: fix memory leaks on `ER_MULTISTATEMENT_TRANSACTION` in DDL

Space index build and space format checking operations don't destroy space
iterator on `txn_check_singlestatement` failure — fix this.

Closes #8773

NO_DOC=bugfix
NO_TEST=<leak happens in small, cannot be detected by sanitizer>

changelogs/unreleased/gh-8733-ddl-memory-leaks.md

+## bugfix/box
+
+* Fixed the memory leaks caused by the multi-statement transaction errors in the
+  space index building and the space format checking operations (gh-8773).

src/box/memtx_space.c

@@ -974,11 +974,11 @@ memtx_space_check_format(struct space *space, struct tuple_format *format)
 	if (index_size(pk) == 0)
 		return 0;
 
-	struct iterator *it = index_create_iterator(pk, ITER_ALL, NULL, 0);
-	if (it == NULL)
+	if (txn_check_singlestatement(txn, "space format check") != 0)
 		return -1;
 
-	if (txn_check_singlestatement(txn, "space format check") != 0)
+	struct iterator *it = index_create_iterator(pk, ITER_ALL, NULL, 0);
+	if (it == NULL)
 		return -1;
 
 	struct memtx_engine *memtx = (struct memtx_engine *)space->engine;
@@ -1224,6 +1224,9 @@ memtx_space_build_index(struct space *src_space, struct index *new_index,
 		return -1;
 	}
 
+	if (txn_check_singlestatement(txn, "index build") != 0)
+		return -1;
+
 	/* Now deal with any kind of add index during normal operation. */
 	struct iterator *it = index_create_iterator(pk, ITER_ALL, NULL, 0);
 	if (it == NULL)
@@ -1237,9 +1240,6 @@ memtx_space_build_index(struct space *src_space, struct index *new_index,
 	 */
 	bool can_yield = pk->def->type != HASH;
 
-	if (txn_check_singlestatement(txn, "index build") != 0)
-		return -1;
-
 	struct memtx_engine *memtx = (struct memtx_engine *)src_space->engine;
 	struct memtx_ddl_state state;
 	struct trigger on_replace;