Fixed the contents of RecoBTag/PerformanceDB/python/measure/Pool_btagMistagWinter13.py

Same as #3164. Bugfix for the problem reported in https://hypernews.cern.ch/HyperNews/CMS/get/btag/1070.html. The bug originally sneaked in with #1364.

In addition, could #2503 be reopened and merged? Also note that this PR and #2503 have zero impact on the standard reconstruction.