pack-bitmap.c: ensure pseudo-merge offset reads are bounded

pack-bitmap.c: ensure pseudo-merge offset reads are bounded

After reading the pseudo-merge extension's metadata table, we allocate
an array to store information about each pseudo-merge, including its
byte offset within the .bitmap file itself.

This is done like so:

    pseudo_merge_ofs = index_end - 24 -
            (index->pseudo_merges.nr * sizeof(uint64_t));
    for (i = 0; i < index->pseudo_merges.nr; i++) {
            index->pseudo_merges.v[i].at = get_be64(pseudo_merge_ofs);
            pseudo_merge_ofs += sizeof(uint64_t);
    }

But if the pseudo-merge table is corrupt, we'll keep calling get_be64()
past the end of the pseudo-merge extension, potentially reading off the
end of the mmap'd region.

Prevent this by ensuring that we have at least `table_size - 24` many
bytes available to read (adding 24 to the left-hand side of our
inequality to account for the length of the metadata component).

This is sufficient to prevent us from reading off the end of the
pseudo-merge extension, and ensures that all of the get_be64() calls
below are in bounds.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>