Fix compiler warning
Fix compiler warning

lib/vssolution3d.cpp: In member function ‘void VisualizationSceneSolution3d::DrawRefinedWedgeLevelSurf(const mfem::DenseMatrix&, const mfem::Vector&, const int*, int, int, const mfem::DenseMatrix*)’:
vssolution3d.cpp:3051:20: warning: ‘l1’ may be used uninitialized in this function [-Wmaybe-uninitialized]
          const int s = l1-l0;
                    ^
vssolution3d.cpp:3055:27: warning: ‘l0’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       const int fsl = ((pk-l0)%2 == 0) ? face_splits : fs2;
                        ~~~^~~~

GLVis/glvis,GLVis/glvis,GLVis/glvis,GLVis/glvis