pack-revindex.c: guard against out-of-bounds pack lookups

pack-revindex.c: guard against out-of-bounds pack lookups

The function midx_key_to_pack_pos() is a helper function used by
midx_to_pack_pos() and midx_pair_to_pack_pos() to translate a (pack,
offset) tuple into a position into the MIDX pseudo-pack order.

Ensure that the pack ID given to midx_pair_to_pack_pos() is bounded by
the number of packs within the MIDX to prevent, for instance,
uninitialized memory from being used as a pack ID.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>