commit-graph: check bounds when accessing BIDX chunk

commit-graph: check bounds when accessing BIDX chunk

We load the bloom_filter_indexes chunk using pair_chunk(), so we have no
idea how big it is. This can lead to out-of-bounds reads if it is
smaller than expected, since we index it based on the number of commits
found elsewhere in the graph file.

We can check the chunk size up front, like we do for CDAT and other
chunks with one fixed-size record per commit.

The test case demonstrates the problem. It actually won't segfault,
because we end up reading random data from the follow-on chunk (BDAT in
this case), and the bounds checks added in the previous patch complain.
But this is by no means assured, and you can craft a commit-graph file
with BIDX at the end (or a smaller BDAT) that does segfault.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>