BUG FIX: compress threaded spawned array correctly.
BUG FIX: compress threaded spawned array correctly.

We were not compressing the threaded spawned array correctly if a thread
did not spawn any particles.  This off-by-one error led to spawning
events set in a previous iteration being re-used if a thread did not
spawn any new particles in the current iteration.

hande-qmc/hande,ruthfranklin/hande,hande-qmc/hande,hande-qmc/hande,hande-qmc/hande,hande-qmc/hande