midx: avoid negative array index

midx: avoid negative array index

nth_midxed_pack_int_id() returns the index of the pack file in the multi
pack index's list of packfiles that the specified object. The index is
returned as a uint32_t. Storing this in an int will make the index
negative if the most significant bit is set. Fix this by using uint32_t
as the rest of the code does. This is unlikely to be a practical problem
as it requires the multipack index to reference 2^31 packfiles.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>