Check whether a ValueAnalysisState contains a MemoryLocation before asking for its value.

This avoids NullPointerExceptions during strengthening of ConstraintsCPA if ValueAnalysis does not track all variables.



git-svn-id: https://svn.sosy-lab.org/software/cpachecker/branches/symbolic@15167 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c