midx: bounds-check large offset chunk

When we see a large offset bit in the regular midx offset table, we use the entry as an index into a separate large offset table (just like a pack idx does). But we don't bounds-check the access to that large offset table (nor even record its size when we parse the chunk!). The equivalent code for a regular pack idx is in check_pack_index_ptr(). But things are a bit simpler here because of the chunked format: we can just check our array index directly. As a bonus, we can get rid of the st_mult() here. If our array bounds-check is successful, then we know that the result will fit in a size_t (and the bounds check uses a division to avoid overflow entirely). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>