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

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

When writing a MIDX, we use the chunk-format API to write out each
individual chunk of the MIDX. Each chunk of the MIDX is tracked via a
call to `add_chunk()`, along with the expected size of that chunk.

Guard against overflow when dealing with a MIDX with a large number of
entries (and consequently, large chunks within the MIDX file itself) to
avoid corrupting the contents of the MIDX itself.

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