avoid zero division if source rate of previous result is zero (#3169) #5

Job	Run time
Python 3.10 (omp=n, mpi=n, dagmc=n, ncrystal=n, libmesh=n, event=n vectfit=n)	49m 59s
Python 3.10 (omp=y, mpi=n, dagmc=n, ncrystal=n, libmesh=n, event=n vectfit=n)	1h 11m 37s
Python 3.10 (omp=n, mpi=y, dagmc=n, ncrystal=n, libmesh=n, event=n vectfit=n)	43m 12s
Python 3.10 (omp=y, mpi=y, dagmc=n, ncrystal=n, libmesh=n, event=n vectfit=n)	1h 5m 26s
Python 3.11 (omp=n, mpi=n, dagmc=, ncrystal=, libmesh=, event= vectfit=)	1h 8m 7s
Python 3.12 (omp=n, mpi=n, dagmc=, ncrystal=, libmesh=, event= vectfit=)	51m 12s
Python 3.10 (omp=y, mpi=y, dagmc=y, ncrystal=, libmesh=, event= vectfit=)	1h 23m 52s
Python 3.10 (omp=n, mpi=n, dagmc=, ncrystal=y, libmesh=, event= vectfit=)	53m 23s
Python 3.10 (omp=y, mpi=y, dagmc=, ncrystal=, libmesh=y, event= vectfit=)	1h 12m 16s
Python 3.10 (omp=y, mpi=n, dagmc=, ncrystal=, libmesh=y, event= vectfit=)	1h 20m 11s
Python 3.10 (omp=y, mpi=n, dagmc=, ncrystal=, libmesh=, event=y vectfit=)	1h 23m 28s
Python 3.10 (omp=n, mpi=y, dagmc=, ncrystal=, libmesh=, event= vectfit=y)	48m 55s
finish	2s
	12h 51m 40s

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