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

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

In a similar spirit as previous patches, avoid an overflow when looking
up object offsets in the MIDX's large offset table by guarding the
computation via `st_mult()`.

This instance is also OK as-is, since the left operand is the result of
`sizeof(...)`, which is already a `size_t`. But use `st_mult()` instead
here to make it explicit that this computation is to be performed using
64-bit unsigned integers.

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