pxh.cpp: fix invalid reference if words is empty
pxh.cpp: fix invalid reference if words is empty

This can happen for example if 'line' is a space.

dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,dagar/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware