Fix division by zero while calculating scheduler score
Fix division by zero while calculating scheduler score

Reported by Eric Wong

yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx