midx.c: prevent overflow in nth_midxed_offset()

In a similar spirit as previous patches, avoid an overflow when looking
up object offsets in the MIDX's large offset table by guarding the
computation via `st_mult()`.

This instance is also OK as-is, since the left operand is the result of
`sizeof(...)`, which is already a `size_t`. But use `st_mult()` instead
here to make it explicit that this computation is to be performed using
64-bit unsigned integers.

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

midx.c

@@ -271,7 +271,8 @@ off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 			die(_("multi-pack-index stores a 64-bit offset, but off_t is too small"));
 
 		offset32 ^= MIDX_LARGE_OFFSET_NEEDED;
-		return get_be64(m->chunk_large_offsets + sizeof(uint64_t) * offset32);
+		return get_be64(m->chunk_large_offsets +
+				st_mult(sizeof(uint64_t), offset32));
 	}
 
 	return offset32;