Don't coredump on bad refs in update-server-info.

Apparently if we are unable to parse an object update-server-info
coredumps, as it doesn't bother to check the return value of its
call to parse_object.

Instead of coredumping, skip the ref.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

server-info.c

@@ -10,6 +10,8 @@ static FILE *info_ref_fp;
 static int add_info_ref(const char *path, const unsigned char *sha1, int flag, void *cb_data)
 {
 	struct object *o = parse_object(sha1);
+	if (!o)
+		return -1;
 
 	fprintf(info_ref_fp, "%s	%s\n", sha1_to_hex(sha1), path);
 	if (o->type == OBJ_TAG) {