Fix crash on join when SSM is preloaded but doesn't have focus.

stepmania/src/Screen.cpp

@@ -42,6 +42,7 @@ void Screen::Init()
 	SetFOV( 0 );
 
 	m_smSendOnPop = SM_None;
+	m_bRunning = false;
 
 	ActorUtil::LoadAllCommandsFromName( *this, m_sName, "Screen" );
 
@@ -68,6 +69,7 @@ void Screen::Init()
 
 void Screen::BeginScreen()
 {
+	m_bRunning = true;
 	m_bFirstUpdate = true;
 
 	/* Screens set these when they determine their next screen dynamically.  Reset them
@@ -87,6 +89,7 @@ void Screen::BeginScreen()
 void Screen::EndScreen()
 {
 	this->PlayCommand( "End" );
+	m_bRunning = false;
 }
 
 void Screen::Update( float fDeltaTime )

stepmania/src/Screen.h

@@ -87,6 +87,9 @@ protected:
 
 	float                           m_fLockInputSecs;
 
+	/* If currently between BeginScreen/EndScreen calls: */
+	bool m_bRunning;
+
 public:
 	RString GetNextScreenName() const;
 	RString GetPrevScreen() const;

stepmania/src/ScreenSelectMusic.cpp

@@ -675,7 +675,7 @@ void ScreenSelectMusic::ChangeDifficulty( PlayerNumber pn, int dir )
 
 void ScreenSelectMusic::HandleMessage( const Message &msg )
 {
-	if( msg == Message_PlayerJoined )
+	if( m_bRunning && msg == Message_PlayerJoined )
 	{
 		// The current steps may no longer be playable.  If one player has double steps 
 		// selected, they are no longer playable now that P2 has joined.