Fix assert fail. (Found this one by letting autoplay go for a while with

the debug console paused; might normally happen in a fast song after
losing a bunch of frames.)

stepmania/src/GrayArrow.cpp

@@ -47,6 +47,7 @@ void GrayArrow::Update( float fDeltaTime )
 void GrayArrow::Step()
 {
 	SetZoom( g_fStepZoom );
+	StopTweening();
 	BeginTweening( g_fStepSeconds );
 	SetTweenZoom( 1 );
 }