Fix crash on exit

Because the map canvas is destroyed before the project instance,
and QgsProject emits the labelingEngineSettingsChange signal
during its destructor, the lambda slot was getting executed
with a dangling map canvas pointer.

Correctly set the receiver in the lambda connection instead
so that the connection gets destroyed along with the canvas.

(cherry-picked from b683291)

Author: nyalldawson

src/app/qgisapp.cpp

@@ -3184,7 +3184,7 @@ void QgisApp::setupConnections()
   } );
 
   connect( QgsProject::instance(), &QgsProject::labelingEngineSettingsChanged,
-           this, [ = ]
+           mMapCanvas, [ = ]
   {
     mMapCanvas->setLabelingEngineSettings( QgsProject::instance()->labelingEngineSettings() );
   } );