FWLite MultiChainEvent: avoid crash on empty files (75X)

[gpetruc]

Forward port of #10739. Tested in CMSSW_7_5_X_2015-08-20-2300 (both that it is needed and that it works)

[cmsbuild]

A new Pull Request was created by @gpetruc (Giovanni Petrucciani) for CMSSW_7_5_X.

FWLite MultiChainEvent: avoid crash on empty files (75X)

It involves the following packages:

DataFormats/FWLite

@cmsbuild, @smuzaffar, @Dr15Jones can you please review it and eventually sign? Thanks.
@wddgit, @wmtan this is something you requested to watch as well.
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@Degano you are the release manager for this.
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[Dr15Jones]

please test

[Dr15Jones]

+1

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_5_X IBs once checked with relvals in the development release cycle of CMSSW or unless it breaks tests. This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_5_X IBs once checked with relvals in the development release cycle of CMSSW or unless it breaks tests. This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[cmsbuild]

-1
Tested at: a7ec3b5
When I ran the RelVals I found an error in the following worklfows:
1000.0 step1

DAS Error

1001.0 step1

DAS Error

1003.0 step1

DAS Error

you can see the results of the tests here:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-10896/7537/summary.html

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_5_X IBs once checked with relvals in the development release cycle of CMSSW (but tests are reportedly failing). This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar