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

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

In a similar spirit as in previous commits, avoid an integer overflow
when computing the expected size of a MIDX.

(Note that this is also OK as-is, since `p->pack_size` is an `off_t`, so
this computation should already be done as 64-bit integers. But again,
let's use `st_mult()` to make this fact clear).

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