Fix warnings from new mass vs z histograms

This is a fix for the warning in opt builds:

/build/jenkins-build/workspace/nightly-builds/build/build/HLT/HLT_2017-patches/Hlt/HltGlobalMonitor/src/HistoWrapper.cpp: In member function 'void HistoWrapper::fill(const LHCb::ODIN*, const LHCb::HltSelReports*)':

/build/jenkins-build/workspace/nightly-builds/build/build/HLT/HLT_2017-patches/Hlt/HltGlobalMonitor/src/HistoWrapper.cpp:213:20: warning: '*((void*)& z +4)' may be used uninitialized in this function [-Wmaybe-uninitialized]