Set spawning rate to 0 if no attempts were made.
Set spawning rate to 0 if no attempts were made.

Avoid a divide-by-zero error.  If no attempts were made, then we can't have created any
new particles and hence rspawn = 0/0 = 0 (for this case, at least).  This allows the
spawning rate to be defined (instead of NaN) even when only some processors are
occupied.

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