Loading a bgzip block can write out of bounds if size overflows.

Affected versions of bam set the length of an internal buffer using
self.compressed.set_len(block_size - HEADER_SIZE - MIN_EXTRA_SIZE) and then
wrote into it. While block_size was constrained to a proper maximum, when it
was too small the subtraction could overflow negatively to a large number past
the capacity of self.compressed.

This can result in memory corruption in the form of writing out of bounds when
loading a bgzip file with a small block_size.

Commit 061eee38d4 fixed this issue by checking for the underflow when setting
the buffer size.