Fix for division by zero in RecoBTag/PerformanceDB: Forward port from 7_2_X to 7_3_X

[ferencek]

Forward port of #5415

[cmsbuild]

A new Pull Request was created by @ferencek (Dinko Ferencek) for CMSSW_7_3_X.

Fix for division by zero in RecoBTag/PerformanceDB: Forward port from 7_2_X to 7_3_X

It involves the following packages:

RecoBTag/PerformanceDB

@cmsbuild, @nclopezo, @StoyanStoynev, @slava77 can you please review it and eventually sign? Thanks.
@ferencek, @acaudron, @pvmulder, @imarches this is something you requested to watch as well.
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@nclopezo, @ktf you are the release manager for this.
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[cmsbuild]

+1
Tested at: 42ef699
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-5448/35/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-5448/35/summary.html

[StoyanStoynev]

+1
For 42ef699 .
Identical to #5415 (for 72X), trivial bug fix - see there. Jenkins here show familiar non-existent features.

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_3_X IBs unless changes (tests are also fine). @nclopezo, @ktf can you please take care of it?