Fix logic bug in if statement
Fixes a logic bug exposed by the following clang compilation warnings:
/build/jenkins-build/workspace/nightly-builds/build/build/ANALYSIS/ANALYSIS_HEAD/Phys/DisplVerticesMC/src/MCDisplVertices.cpp:3361:14: warning: taking the absolute value of unsigned type 'bool' has no effect [-Wabsolute-value]
/build/jenkins-build/workspace/nightly-builds/build/build/ANALYSIS/ANALYSIS_HEAD/Phys/DisplVerticesMC/src/MCDisplVertices.cpp:3361:48: warning: taking the absolute value of unsigned type 'bool' has no effect [-Wabsolute-value]```