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

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>