Simply limiting the lookahead region to the size of
the block device fixes the problem.
Also added logic to limit the allocated region and
floor to nearest word, since the additional memory
couldn't really be used effectively.
When the lookahead buffer wraps around in an unaligned filesystem, it's
possible for blocks at the beginning of the disk to have a negative distance
from the lookahead, but still reside in the lookahead buffer.
Switching to signed modulo doesn't quite work due to how negative modulo
is implemented in C, so the simple solution is to shift the region to be
positive.
