commit-graph: bounds-check generation overflow chunk

If the generation entry in a commit-graph doesn't fit, we instead insert an offset into a generation overflow chunk. But since we don't record the size of the chunk, we may read outside the chunk if the offset we find on disk is malicious or corrupted. We can't check the size of the chunk up-front; it will vary based on how many entries need overflow. So instead, we'll do a bounds-check before accessing the chunk memory. Unfortunately there is no error-return from this function, so we'll just have to die(), which is what it does for other forms of corruption. As with other cases, we can drop the st_mult() call, since we know our bounds-checked value will fit within a size_t. Before this patch, the test here actually "works" because we read garbage data from the next chunk. And since that garbage data happens not to provide a generation number which changes the output, it appears to work. We could construct a case that actually segfaults or produces wrong output, but it would be a bit tricky. For our purposes its sufficient to check that we've detected the bounds error. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>