commit-graph.c: prevent overflow in `load_oid_from_graph()`

commit-graph.c: prevent overflow in `load_oid_from_graph()`

In a similar spirit as previous commits, ensure that we don't overflow
when trying to compute an offset into the `chunk_oid_lookup` table when
the `lex_index` of the item we're trying to look up exceeds
`2^32-1/g->hash_len`.

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