macosx: fix another crash on exit

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

modules/gui/macosx/ExtensionsManager.m

@@ -352,7 +352,8 @@ static ExtensionsManager* instance = nil;
     msg_Dbg( p_intf, "Killing extension dialog provider" );
     [ExtensionsDialogProvider killInstance];
 
-    vlc_object_release( p_extensions_manager );
+    if( p_extensions_manager )
+        vlc_object_release( p_extensions_manager );
 
     [p_extDict release];