unload BGA for screens that don't have a background

stepmania/src/ScreenManager.cpp

@@ -355,10 +355,15 @@ retry:
 	}
 	
 	// Load shared BGAnimation
+	CString sNewBGA;
 	if( pNewScreen->UsesBackground() )
+		sNewBGA = THEME->GetPathToB(sScreenName+" background");
+	if( sNewBGA.empty() )
 	{
-		CString sNewBGA = THEME->GetPathToB(sScreenName+" background");
-		if( m_sLastLoadedBackground != sNewBGA )
+		m_pSharedBGA->Unload();
+		m_sLastLoadedBackground = "";
+	}
+	else if( m_sLastLoadedBackground != sNewBGA )
 	{
 		// Create the new background before deleting the previous so that we keep
 		// any common textures loaded.
@@ -369,7 +374,6 @@ retry:
 
 		m_sLastLoadedBackground = sNewBGA;
 	}
-	}	
 
 	bool bWasOnSystemMenu = GAMESTATE->m_bIsOnSystemMenu;