verify_packfile: check pack validity before accessing data

The verify_packfile() does not explicitly open the packfile; instead, it starts with a sha1 checksum over the whole pack, and relies on use_pack() to open the packfile as a side effect. If the pack cannot be opened for whatever reason (either because its header information is corrupted, or perhaps because a simultaneous repack deleted it), then use_pack() will die(), as it has no way to return an error. This is not ideal, as verify_packfile() otherwise tries to gently return an error (this lets programs like git-fsck go on to check other packs). Instead, let's check is_pack_valid() up front, and return an error if it fails. This will open the pack as a side effect, and then use_pack() will later rely on our cached descriptor, and avoid calling die(). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>