[Android] Fix potential NPE

[YorkShen]

java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.next(ArrayList.java:860)
at com.taobao.weex.ui.view.WXHorizontalScrollView.onScrollChanged(WXHorizontalScrollView.java:63)
at android.view.View.scrollTo(View.java:15286)
at android.widget.HorizontalScrollView.scrollTo(HorizontalScrollView.java:1635)
at android.view.View.scrollBy(View.java:15301)
at com.taobao.weex.ui.component.WXScroller$12.run(WXScroller.java:852)
at android.os.Handler.handleCallback(Handler.java:808)
at android.os.Handler.dispatchMessage(Handler.java:101)
at android.os.Looper.loop(Looper.java:166)
at android.app.ActivityThread.main(ActivityThread.java:7425)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
Back traces end.

[lucky-chen]

LGTM