course timed attack support

fix HowToPlay crash

stepmania/src/Attack.cpp

@@ -10,6 +10,11 @@ void Attack::GetAttackBeats( const Song *song, PlayerNumber pn, float &fStartBea
 		fStartBeat = song->GetBeatFromElapsedTime( fStartSecond );
 		fEndBeat = song->GetBeatFromElapsedTime( fStartSecond+fSecsRemaining );
 	} else {
+		/* If fStartSecond < 0, then the attack starts right off the screen; this requires
+		 * that a song actually be playing.  Pre-queued course attacks must always have 
+		 * fStartSecond >= 0. */
+		ASSERT( GAMESTATE->m_pCurSong );
+		
 		/* We're setting this effect on the fly.  If it's an arrow-changing effect
 		 * (transform or note skin), apply it in the future, past what's currently on
 		 * screen, so new arrows will scroll on screen with this effect. */