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

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