line-log: handle deref_tag() returning NULL

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

line-log.c

@@ -481,7 +481,7 @@ static struct commit *check_single_commit(struct rev_info *revs)
 		if (obj->flags & UNINTERESTING)
 			continue;
 		obj = deref_tag(revs->repo, obj, NULL, 0);
-		if (obj->type != OBJ_COMMIT)
+		if (!obj || obj->type != OBJ_COMMIT)
 			die("Non commit %s?", revs->pending.objects[i].name);
 		if (commit)
 			die("More than one commit to dig from: %s and %s?",