lost code that handled the case of attempting to merge a zero length
index dropping file.   zero length index dropping files can happen
after a ftruncate() to zero.

in this case we don't want to call GetDataBuf() since that fails
if len == 0.   instead, we just set the buffer to null (and the
0 length will drop us through to the end --- because merging a
zero length dropping is a no-op).