[LinuxCNC/linuxcnc PR#10] tp: possible fix for non-zero current velocity when stopped

tpHandleEmptyQueue now explicitly sets status flags in tp and
emcmotStatus to the appropriate defaults. This should prevent spurious
non-zero values from appearing in current_vel and similar status fields
when the motion queue is empty. This was likely an overlooked regression
from 2.6 due to the major refactor introduced with the new trajectory
planner.