stash: defensive programming

CodeQL points out that `lookup_commit()` can return NULL values.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/stash.c

@@ -1461,6 +1461,11 @@ static int do_create_stash(const struct pathspec *ps, struct strbuf *stash_msg_b
 		goto done;
 	} else {
 		head_commit = lookup_commit(the_repository, &info->b_commit);
+		if (!head_commit) {
+			ret = error(_("could not look up commit '%s'"),
+				    oid_to_hex (&info->b_commit));
+			goto done;
+		}
 	}
 
 	if (!check_changes(ps, include_untracked, &untracked_files)) {