midx.c: store `nr`, `alloc` variables as `size_t`'s

midx.c: store `nr`, `alloc` variables as `size_t`'s

In the `write_midx_context` structure, we use two `uint32_t`'s to track
the length and allocated size of the packs, and one `uint32_t` to track
the number of objects in the MIDX.

In practice, having these be 32-bit unsigned values shouldn't cause any
problems since we are unlikely to have that many objects or packs in any
real-world repository. But these values should be `size_t`'s, so change
their type to reflect that.

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