midx.c: prevent overflow in `nth_midxed_object_oid()`

midx.c: prevent overflow in `nth_midxed_object_oid()`

In a similar spirit as previous commits, avoid overflow when looking up
an object's OID in a MIDX when its position is greater than
`2^32-1/m->hash_len`.

As usual, it is perfectly OK for a MIDX to have as many as 2^32-1
objects (since we use 32-bit fields to count the number of objects at
each fanout layer). But if we have more than `2^32-1/m->hash_len` number
of objects, we will incorrectly perform the computation using 32-bit
integers, overflowing the result.

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