CancelStage when backing out of ScreenStage. This fixes backing out of final stage early goes to extra stage and backing out of extra stage early goes to extra stage 2.

stepmania/src/ScreenStage.cpp

@@ -107,6 +107,7 @@ void ScreenStage::MenuBack( PlayerNumber pn )
 		return;
 	
 	this->ClearMessageQueue();
+	GAMESTATE->CancelStage();
 	m_sNextScreen = GetPrevScreen();
 	m_Cancel.StartTransitioning( SM_GoToPrevScreen );