Don't die in git-http-fetch when fetching packs.

Don't die in git-http-fetch when fetching packs.

My sp/mmap changes to pack-check.c modified the function such that
it expects packed_git.pack_size to be populated with the total
bytecount of the packfile by the caller.

But that isn't the case for packs obtained by git-http-fetch as
pack_size was not initialized before being accessed.  This caused
verify_pack to think it had 2^32-21 bytes available when the
downloaded pack perhaps was only 305 bytes in length.  The use_pack
function then later dies with "offset beyond end of packfile"
when computing the overall file checksum.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>