Don't dereference null pointer

[wmtan]

This is a trivial fix for a segfault in PoolSource. If secondaryFileNames are specified but not used, the secondary file names are cleared, but then the resulting nullptr is dereferenced. The fix is trivial and obvious. The segfault was observed by Matti Kortelainen.
This fix can easily be backported to prior releases should it become needed.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_8_0_X.

It involves the following packages:

IOPool/Input

@cmsbuild, @smuzaffar, @Dr15Jones, @davidlange6 can you please review it and eventually sign? Thanks.
@wddgit this is something you requested to watch as well.
@slava77, @Degano, @smuzaffar you are the release manager for this.

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[wmtan]

@makortel This fixes the segfault you saw.

[Dr15Jones]

please test

[Dr15Jones]

+1

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/9886/console

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_8_0_X IBs after it passes the integration tests. This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar

[cmsbuild]

+1
Tested at: 0ce4c6d
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12519/9886/summary.html

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_8_0_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12519/9886/summary.html

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_8_0_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar

[davidlange6]

+1