Fix LSC crash, guard against invalid label index.

modules/ximgproc/src/lsc.cpp

@@ -768,6 +768,8 @@ inline void SuperpixelLSCImpl::PostEnforceLabelConnectivity( int threshold )
 
       double W = W1 + W2;
 
+      if ( (Label1 > 0) && (Label2 > 0) )
+      {
         for ( int b = 0; b < m_nr_channels; b++ )
         {
           centerC1[b][Label2] = float((W2*centerC1[b][Label2] + W1*centerC1[b][Label1]) / W);
@@ -785,6 +787,7 @@ inline void SuperpixelLSCImpl::PostEnforceLabelConnectivity( int threshold )
           int y = (*S).yLoc[i];
           m_klabels.at<int>(y,x) = Label2;
         }
+      }
 
       vector<Superpixel>::iterator Stmp;
       Stmp = find( Sarray.begin(), Sarray.end(), Label2 );