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

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

In a similar spirit as previous commits, ensure that we don't overflow
when the lex_index of the commit we are trying to fill out exceeds
2^32-1/(g->hash_len+16).

The other hunk touched in this patch is not susceptible to overflow,
since an explicit cast is made to a 64-bit unsigned value. For clarity
and consistency with the rest of the commits in this series, avoid a
tricky to reason about cast, and use `st_mult()` directly.

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