sha1fd_check: die when we cannot open the file

sha1fd_check: die when we cannot open the file

Right now we return a NULL "struct sha1file" if we encounter
an error. However, the sole caller (write_idx_file) does not
check the return value, and will segfault if we hit this
case.

One option would be to handle the error in the caller.
However, there's really nothing for it to do but die. This
code path is hit during "git index-pack --verify"; after we
verify the packfile, we check that the ".idx" we would
generate from it is byte-wise identical to what is on disk.
We hit the error (and segfault) if we can't open the .idx
file (a likely cause of this is that somebody else ran "git
repack -ad" while we were verifying). Since we can't
complete the requested verification, we really have no
choice but to die.

Furthermore, the rest of the sha1fd_* functions simply die
on errors. So if were to open the file successfully, for
example, and then hit a read error, sha1write would call
die() for us. So pushing the die() down into sha1fd_check
keeps the interface consistent.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>