http-walker: verify remote packs

http-walker: verify remote packs

In c17fb6e ("Verify remote packs, speed up pending request queue"),
changes were made to index fetching in http-push.c, particularly the
methods fetch_index and setup_index. Since http-walker.c has similar
code for index fetching, these improvements should apply to
http-walker.c's fetch_index and setup_index.

Invocations of free() of string memory are reproduced as well.

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