midx.c: fix an integer underflow

midx.c: fix an integer underflow

When verifying a midx index with 0 objects, the
    m->num_objects - 1
underflows and wraps around to 4294967295.

Fix this both by checking that the midx contains at least one oid,
and also that we don't write any midx when there is no packfiles.

Update the tests to check that `git multi-pack-index write` does
not write an midx when there is no objects, and another to check
that `git multi-pack-index verify` warns when it verifies an midx with no
objects. For this last test, use t5319/no-objects.midx which was
generated by an older version of git.

Signed-off-by: Damien Robert <redacted>
Signed-off-by: Junio C Hamano <redacted>