[4.0] Text auto scroll animation bug fix

[4.0] Text auto scroll animation bug fix

 Before this patch,
 That code played scroll animation.
    textLabel = TextLabel::New( "Hello World" );
    textLabel.SetProperty( Toolkit::TextLabel::Property::AUTO_SCROLL_STOP_MODE, TextLabel::AutoScrollStopMode::IMMEDIATE);
    textLabel.SetProperty( Toolkit::TextLabel::Property::ENABLE_AUTO_SCROLL,true);
    textLabel.SetProperty( Toolkit::TextLabel::Property::ENABLE_AUTO_SCROLL,false);
    stage.Add(textLabel);

 So I added code for stopping animation when mScrollAnimation was Null.

Change-Id: I4fa881215c4c30228e2cb0fb6f9f80e7060bf6b2