fix div-by-0 error
fix div-by-0 error

Winterflower/khmer,jas14/khmer,F1000Research/khmer,souravsingh/khmer,ged-lab/khmer,ged-lab/khmer,kdmurray91/khmer,Winterflower/khmer,ged-lab/khmer,kdmurray91/khmer,jas14/khmer,F1000Research/khmer,Winterflower/khmer,kdmurray91/khmer,F1000Research/khmer,jas14/khmer,souravsingh/khmer,souravsingh/khmer