fix pat::Electron source PFCand accessor in miniAOD

[gpetruc]

References from pat::Electron to packed PFCandidates are correctly stored in the miniAODs, but the accessor that returns them had a bug.
This commit fixes the problem in the accessor.
It does not need to reproduce the miniAODs, it does not change the persistent dataformat, and in general it doesn't affect miniAOD production in any way.
It also doesn't need to recompile any dependency for it to work, since the fix is localized in a cc file.

@arizzi @bendavid

[cmsbuild]

A new Pull Request was created by @gpetruc (Giovanni Petrucciani) for CMSSW_7_0_X.

fix pat::Electron source PFCand accessor in miniAOD

It involves the following packages:

DataFormats/PatCandidates

@nclopezo, @vadler, @cmsbuild, @Degano, @monttj can you please review it and eventually sign? Thanks.
You can sign-off by replying to this message having '+1' in the first line of your reply.
You can reject by replying to this message having '-1' in the first line of your reply.
@nclopezo, @ktf you are the release manager for this.
You can merge this pull request by typing 'merge' in the first line of your comment.

[cmsbuild]

Pull request #4862 was updated. @nclopezo, @vadler, @cmsbuild, @Degano, @monttj can you please check and sign again.

[cmsbuild]

+1
Tested at: 764ac04
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-4862/242/summary.html

[monttj]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_0_X IBs unless changes (tests are also fine). @nclopezo, @ktf can you please take care of it?