Fix a very rare crash in GHCi

Fix a very rare crash in GHCi
When a BCO with a zero-length bitmap was right at the edge of
allocated memory, we were reading a word of non-existent memory.

This showed up as a segfault in T789(ghci) for me, but the crash was
extremely sensitive and went away with most changes.

Also, optimised scavenge_large_bitmap a bit while I was in there.