http-push: do not SEGV after fetching a bad pack idx file

In a70c232 ("http-fetch: do not SEGV after fetching a bad pack idx
file"), changes were made to the setup_index method in http-fetch.c
(known in its present form as http-walker.c after 30ae764 ("Modularize
commit-walker")). Since http-push.c has similar similar code for
processing index files, these changes should apply to http-push.c's
implementation of setup_index as well.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http-push.c

@@ -1065,6 +1065,8 @@ static int setup_index(unsigned char *sha1)
 		return -1;
 
 	new_pack = parse_pack_index(sha1);
+	if (!new_pack)
+		return -1; /* parse_pack_index() already issued error message */
 	new_pack->next = repo->packs;
 	repo->packs = new_pack;
 	return 0;