Fix issue causing reading v0.8 blockfiles

When skipping nul bytes at the end of the *LAST* block in a file, we
should store the start of the nul span in blkfile_offset, not the end.

It appears bitcoind v0.8 uses mmap now and allocates the space before
writing to it. See http://github.com/bitcoin/bitcoin commits 5e650d6d
and 4786302f.