Fix crash in HcalDigisClient (again)

This bug shows up as a seg fault in HcalDigisClient during harvesting:

    #5  0x00002b7c734c1a74 in MonitorElement::Fill(double, double) () from /afs/cern.ch/cms/sw/ReleaseCandidates/vol0/slc5_amd64_gcc472/cms/cmssw/CMSSW_6_2_X_SLHC_2014-03-18-0200/lib/slc5_amd64_gcc472/libDQMServicesCore.so
    #6  0x00002b7c831fbcb4 in HcalDigisClient::HcalDigisEndjob(std::vector<MonitorElement*, std::allocator<MonitorElement*> > const&, std::string) () from /afs/cern.ch/cms/sw/ReleaseCandidates/vol0/slc5_amd64_gcc472/cms/cmssw/CMSSW_6_2_X_SLHC_2014-03-18-0200/lib/slc5_amd64_gcc472/pluginValidationHcalDigis.so
    #7  0x00002b7c831fc8d8 in HcalDigisClient::runClient() () from /afs/cern.ch/cms/sw/ReleaseCandidates/vol0/slc5_amd64_gcc472/cms/cmssw/CMSSW_6_2_X_SLHC_2014-03-18-0200/lib/slc5_amd64_gcc472/pluginValidationHcalDigis.so
    #8  0x00002b7c64b7d330 in edm::EDAnalyzer::doEndRun(edm::RunPrincipal const&, edm::EventSetup const&, edm::CurrentProcessingContext const*) () from /afs/cern.ch/cms/sw/ReleaseCandidates/vol0/slc5_amd64_gcc472/cms/cmssw/CMSSW_6_2_X_SLHC_2014-03-18-0200/lib/slc5_amd64_gcc472/libFWCoreFramework.so

Histograms are only booked if an un-intialised boolean is true.  The histograms are filled if the boolean is true, but this is after the variable has been properly initialised.  Hence there is a 50/50 chance that the code will either run as expected or seg fault, depending on the previous memory contents of the variable.
This was previously fixed in #2483 but was then broken again by #2833.