fix crash when maptool of a plugin is active on exit

Author: jef-n

src/gui/qgsmaptool.cpp

@@ -98,7 +98,16 @@ void QgsMapTool::deactivate()
 
 void QgsMapTool::setAction( QAction* action )
 {
+  if( mAction )
+    disconnect( mAction, SIGNAL( destroyed() ), this, SLOT( actionDestroyed() ) );
   mAction = action;
+  connect( mAction, SIGNAL( destroyed() ), this, SLOT( actionDestroyed() ) );
+}
+
+void QgsMapTool::actionDestroyed()
+{
+  if( mAction == sender() )
+    mAction = 0;
 }
 
 QAction* QgsMapTool::action()

src/gui/qgsmaptool.h

@@ -153,6 +153,10 @@ class GUI_EXPORT QgsMapTool : public QObject
     //! emit signal to clear previous message
     void messageDiscarded();
 
+  private slots:
+    //! clear pointer when action is destroyed
+    void actionDestroyed();
+
   protected:
 
     //! constructor takes map canvas as a parameter