Chipper: Fix crash for empty input buffer, rm unused var

Howard, thanks a lot for pointing me to PDAL's Chipper, it was precisely what I needed for processing LAS files.

Below I am fixing a minor segfault issue if the input buffer is empty. The crash is in Chipper::DecideSplit() at the line:

v1range = v1[right].m_pos - v1[left].m_pos;

I also removed an unused variable. Hopefully this could be of help.