Clear Film Simulation combo when changing images

When switching to the "next" image whose HaldCLUT does not exist,
RT would show the previous image HaldCLUT's name in the
"Film Simulation" combo, even though it is not being used.
This patch by Hombre fixes it, and fixes #4388

rtgui/filmsimulation.cc

@@ -425,6 +425,8 @@ void ClutComboBox::setSelectedClut( Glib::ustring filename )
 
         if ( found ) {
             set_active( found );
+        } else {
+            set_active(-1);
         }
     }
 }