Fix bug in EWAH find_next.

When the bit index of the requested offset falls after the last dirty block, we
also need to inspect the first bits:

        +-----------+--------------------+
        |   empty   |     valid bits     |
        +-----------+--------------------+
                    \_____ block len ___/

If the the index falls in the "empty" range, we need to use the first bit
in the valid bits as our next hit of find_next.
Fix problem in asking for segments of hits.