Fix segmentation fault in SuperpixelLSCImpl::PostEnforceLabelConnectivity

See https://github.com/opencv/opencv_contrib/issues/1048

modules/ximgproc/src/lsc.cpp

@@ -792,10 +792,25 @@ inline void SuperpixelLSCImpl::PostEnforceLabelConnectivity( int threshold )
       {
         Size[Label2] = Size[Label1] + Size[Label2];
         if( Size[Label2] >= threshold )
+        {
+          if( S == Stmp )
+          {
+            // erasing only one should be sufficient when the iterators are the same
+            // (maybe the case S == Stmp is not even possible?)
+            Sarray.erase( S );
+          }
+          else if( S < Stmp )
+          {
+            // erase the latter element first, so the other iterator is not invalidated
+            Sarray.erase( Stmp );
+            Sarray.erase( S );
+          }
+          else
           {
             Sarray.erase( S );
             Sarray.erase( Stmp );
           }
+        }
         else
         {
           (*Stmp).xLoc.insert( (*Stmp).xLoc.end(), (*S).xLoc.begin(), (*S).xLoc.end() );