fix crash going from gameplay to how to play with delayed screen loads off

stepmania/src/ScreenGameplay.cpp

@@ -1146,8 +1146,15 @@ void ScreenGameplay::Update( float fDeltaTime )
 	else
 		Screen::Update( fDeltaTime );
 
+	/* This happens if ScreenDemonstration::HandleScreenMessage sets a new screen when
+	 * PREFSMAN->m_bDelayedScreenLoad. */
 	if( GAMESTATE->m_pCurSong == NULL )
 		return;
+	/* This can happen if ScreenDemonstration::HandleScreenMessage sets a new screen when
+	 * !PREFSMAN->m_bDelayedScreenLoad.  (The new screen was loaded when we called Screen::Update,
+	 * and the ctor might set a new GAMESTATE->m_pCurSong, so the above check can fail.) */
+	if( SCREENMAN->GetTopScreen() != this )
+		return;
 
 	if( GAMESTATE->m_MasterPlayerNumber != PLAYER_INVALID )
 		m_MaxCombo.SetText( ssprintf("%d", g_CurStageStats.iMaxCombo[GAMESTATE->m_MasterPlayerNumber]) ); /* MAKE THIS WORK FOR BOTH PLAYERS! */

stepmania/src/ScreenManager.cpp

@@ -371,6 +371,13 @@ void ScreenManager::EmptyDeleteQueue()
 	TEXTUREMAN->DiagnosticOutput();
 }
 
+Screen *ScreenManager::GetTopScreen()
+{
+	if( m_ScreenStack.empty() )
+		return NULL;
+	return m_ScreenStack[m_ScreenStack.size()-1];
+}
+
 void ScreenManager::Update( float fDeltaTime )
 {
 	// Only update the topmost screen on the stack.
@@ -392,7 +399,7 @@ void ScreenManager::Update( float fDeltaTime )
 
 	if( !m_ScreenStack.empty() )
 	{
-		Screen* pScreen = m_ScreenStack[m_ScreenStack.size()-1];
+		Screen* pScreen = GetTopScreen();
 		if( pScreen->IsFirstUpdate() )
 			pScreen->Update( 0 );
 		else

stepmania/src/ScreenManager.h

@@ -59,6 +59,8 @@ public:
 
 	void LoadDelayedScreen();
 
+	Screen *GetTopScreen();
+
 private:
 	vector<Screen*> m_ScreenStack;	// bottommost to topmost
 	ScreenMessage m_MessageSendOnPop;