Don't expect verify_pack() callers to set pack_size

Don't expect verify_pack() callers to set pack_size

Since use_pack() will end up populating pack_size if it is not already set,
we can just adapt the code in verify_packfile() such that it doesn't require
pack_size to be set beforehand.

This allows callers not to have to set pack_size themselves, and we can thus
revert changes from 1c23d794 (Don't die in git-http-fetch when fetching packs).

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