midx.c: use `size_t`'s for fanout nr and alloc

midx.c: use `size_t`'s for fanout nr and alloc

The `midx_fanout` struct is used to keep track of a set of OIDs
corresponding to each layer of the MIDX's fanout table. It stores an
array of entries, along with the number of entries in the table, and the
allocated size of the array.

Both `nr` and `alloc` are stored as 32-bit unsigned integers. In
practice, this should never cause any problems, since most packs have
far fewer than 2^32-1 objects.

But storing these as `size_t`'s is more appropriate, and prevents us
from accidentally overflowing some result when multiplying or adding to
either of these values. Update these struct members to be `size_t`'s as
appropriate.

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