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

In a similar spirit as previous commits, ensure that we don't overflow in a few spots within `fill_commit_graph_info()`: - First, when computing an offset into the commit data chunk, which can occur when the `lex_index` of the item we're looking up exceeds 2^32-1/GRAPH_DATA_WIDTH. - A similar issue when computing the generation date offset for commits with `lex_index` greater than 2^32-1/4. Note that in practice this will never overflow, since the left-hand operand is from calling `sizeof(...)` and is thus already a `size_t`. But wrap that in an `st_mult()` to make it clear that we intend to perform this computation using 64-bit operands. - Finally, a nearly identical issue as above when computing an offset into the `generation_data_overflow` chunk. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>