added check to prevent division by zero
Merge pull request #5448 from cms-btv-pog/FixDivisionByZeroInRecoBTagPerformanceDB_from-CMSSW_7_2_0_pre6

Fix for division by zero in RecoBTag/PerformanceDB: Forward port from 7_2_X to 7_3_X
Merge remote branch 'origin/CMSSW_7_3_X' into CMSSW_7_0_X