Avoid crash from storage of potentially removed map layer storage in canvas

(cherry-picked from 56b6ebd)

Author: nyalldawson

src/gui/qgsmapcanvas.cpp

@@ -285,6 +285,9 @@ QgsMapLayer *QgsMapCanvas::layer( int index )
 
 void QgsMapCanvas::setCurrentLayer( QgsMapLayer *layer )
 {
+  if ( mCurrentLayer == layer )
+    return;
+
   mCurrentLayer = layer;
   emit currentLayerChanged( layer );
 }

src/gui/qgsmapcanvas.h

@@ -917,7 +917,7 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
     bool mRenderFlag = true;
 
     //! current layer in legend
-    QgsMapLayer *mCurrentLayer = nullptr;
+    QPointer< QgsMapLayer > mCurrentLayer;
 
     //! graphics scene manages canvas items
     QGraphicsScene *mScene = nullptr;