Catch errors when writing an index that contains invalid objects.

If git-write-index is called without --missing-ok, it reports invalid
objects that it finds in the index. But without this patch it dies
right away or may run into an infinite loop.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <junkio@cox.net>

cache-tree.c

@@ -282,6 +282,8 @@ static int update_one(struct cache_tree *it,
 				    baselen + sublen + 1,
 				    missing_ok,
 				    dryrun);
+		if (subcnt < 0)
+			return subcnt;
 		i += subcnt - 1;
 		sub->used = 1;
 	}