PNS: Correct assertion when dragging

Dragging using highlight caused an assertion as the duplicated lines
were being re-added with their old links still inplace.  While this was
probably fine as the link references shouldn't be stale, we clear them
to allow for the possibibility that the dragger will adjust the links in
the future.

pcbnew/router/pns_dragger.cpp

@@ -206,6 +206,7 @@ bool DRAGGER::dragMarkObstacles( const VECTOR2I& aP )
         LINE origLine( m_draggedLine );
         LINE dragged( m_draggedLine );
         dragged.SetSnapThreshhold( thresh );
+        dragged.ClearSegmentLinks();
 
         if( m_mode == DM_SEGMENT )
             dragged.DragSegment( aP, m_draggedSegmentIndex );