macosx: fix another crash on exit

(for whatever reason this was only observable on snow leopard for me)

modules/gui/macosx/ExtensionsManager.m

@@ -352,6 +352,7 @@ static ExtensionsManager* instance = nil;
     msg_Dbg( p_intf, "Killing extension dialog provider" );
     [ExtensionsDialogProvider killInstance];
 
+    if( p_extensions_manager )
         vlc_object_release( p_extensions_manager );
 
     [p_extDict release];