fix a bug may throw exception when draw danmuku

报错信息如下
java.lang.RuntimeException: Canvas: trying to use a recycled bitmap android.graphics.Bitmap@4290fee
at android.graphics.Canvas.throwIfRecycled(Canvas.java:1026)
at android.graphics.Canvas.drawBitmap(Canvas.java:1065)
at master.flame.danmaku.danmaku.model.android.AndroidDisplayer.draw(Unknown Source)
at master.flame.danmaku.danmaku.model.BaseDanmaku.draw(Unknown Source)
at master.flame.danmaku.danmaku.renderer.android.DanmakuRenderer.draw(Unknown Source)
at master.flame.danmaku.controller.DrawTask.drawDanmakus(Unknown Source)
at master.flame.danmaku.controller.DrawTask.draw(Unknown Source)
at master.flame.danmaku.controller.CacheManagingDrawTask.draw(Unknown Source)
at master.flame.danmaku.controller.DrawHandler.draw(Unknown Source)
at master.flame.danmaku.ui.widget.DanmakuSurfaceView.drawDanmakus(Unknown Source)
at master.flame.danmaku.controller.DrawHandler.handleMessage(Unknown Source)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:153)
at android.os.HandlerThread.run(HandlerThread.java:60)

AndroidDisplayer 200行
if (holder != null && holder.bitmap != null )->
if (holder != null && holder.bitmap != null && !holder.bitmap.isRecycled())