Fixed navradio when shutting down/restarting nav receiver.

"nav-loc" and "has_gs" properties were not updated when nav receiver was rebooted.

Shutting down the nav receiver clears all nav outputs (including "nav-loc" and "has_gs").
=> When nav receiver is powered again, all outputs must be updated.
=> "nav-loc" and "has_gs" are only updated when active nav station changes.
=> old nav station must be cleared on shutdown to enforce update on nav reboot...

src/Instrumentation/navradio.cxx

@@ -390,6 +390,7 @@ void FGNavRadio::clearOutputs()
   
   _dmeInRange = false;
   _operable = false;
+  _navaid = NULL;
 }
 
 void FGNavRadio::updateReceiver(double dt)