Bugfix: SSM: Make buffers long enough to avoid overflows

- coinb2 was missing room for the trailing \0 byte
- In the rare event of an empty merkle link list, the length of the serialised list was being treated as -1
Bugfix: SSM: When n2pad<0, release lock before returning
Merge commit 'd1cf622' into bfgminer-4.7.x