packfile.c: use checked arithmetic in `nth_packed_object_offset()`

packfile.c: use checked arithmetic in `nth_packed_object_offset()`

In a similar spirit as the previous commits, ensure that we use
`st_add()` or `st_mult()` when computing values that may overflow the
32-bit unsigned limit.

Note that in each of these instances, we prevent 32-bit overflow
already since we have explicit casts to `size_t`.

So this code is OK as-is, but let's clarify it by using the `st_xyz()`
helpers to make it obvious that we are performing the relevant
computations using 64 bits.

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