pack-revindex.c: don't close unopened file descriptors

pack-revindex.c: don't close unopened file descriptors

When opening a reverse index, load_revindex_from_disk() jumps to the
'cleanup' label in case something goes wrong: the reverse index had the
wrong size, an unrecognized version, or similar.

It also jumps to this label when the reverse index couldn't be opened in
the first place, which will cause an error with the unguarded close()
call in the label.

Guard this call with "if (fd >= 0)" to make sure that we have a valid
file descriptor to close before attempting to close it.

Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>