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>

midx.h

@@ -37,6 +37,7 @@ struct multi_pack_index {
 	const unsigned char *chunk_oid_lookup;
 	const unsigned char *chunk_object_offsets;
 	const unsigned char *chunk_large_offsets;
+	size_t chunk_large_offsets_len;
 	const unsigned char *chunk_revindex;
 
 	const char **pack_names;