Fix -Wtautological-undefined-compare in MeshMath

[jcfr]

/Volumes/Dashboards/Stable/SSALT-0-build/SPHARM-PDM/Modules/CLI/MetaMeshTools/MeshMath.cxx:3744:48: warning: reference cannot be bound to dereferenced null pointer in
      well-defined C++ code; comparison may be assumed to always evaluate to true [-Wtautological-undefined-compare]
      if( inputMesh->GetPoint( i, point1 ) && &Xval[i] != NULL )
                                               ^~~~~~~    ~~~~