Sporadic crash from invalid touch event
Sporadic crash from invalid touch event

When embedded by the view_manager, sometimes we receive pointerup or
pointercancel events without having received a cooresponding pointerdown event.
The underlying issue is that the view_manager doesn't capture on pointerdown
and instead performs a new hit test for every pointer event. We should fix that
in view_manager, but, in the meantime, this patch makes us not crash in this
scenario.

Fixes #339

mikejurka/engine,rmacnak-google/engine,aam/engine,jason-simmons/sky_engine,jamesr/sky_engine,abarth/sky_engine,krisgiesing/sky_engine,jamesr/flutter_engine,flutter/engine,mikejurka/engine,flutter/engine,iansf/sky_engine,mpcomplete/flutter_engine,iansf/sky_engine,tvolkert/engine,aam/engine,devoncarew/sky_engine,jamesr/sky_engine,rmacnak-google/engine,iansf/sky_engine,chinmaygarde/flutter_engine,devoncarew/engine,abarth/sky_engine,xunmengfeng/engine,jamesr/flutter_engine,aam/engine,flutter/engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,axinging/sky_engine,iansf/sky_engine,rmacnak-google/engine,flutter/engine,tvolkert/engine,jason-simmons/flutter_engine,rmacnak-google/engine,abarth/sky_engine,lyceel/engine,tvolkert/engine,krisgiesing/sky_engine,jason-simmons/sky_engine,aam/engine,jason-simmons/sky_engine,devoncarew/engine,jamesr/sky_engine,TribeMedia/sky_engine,qiankunshe/sky_engine,mikejurka/engine,mpcomplete/flutter_engine,Hixie/sky_engine,cdotstout/sky_engine,jamesr/flutter_engine,aam/engine,mpcomplete/flutter_engine,iansf/sky_engine,tvolkert/engine,mikejurka/engine,mikejurka/engine,zhangxq5012/sky_engine,jamesr/flutter_engine,tvolkert/engine,flutter/engine,cdotstout/sky_engine,flutter/engine,devoncarew/engine,devoncarew/engine,krisgiesing/sky_engine,mpcomplete/flutter_engine,mikejurka/engine,afandria/sky_engine,lyceel/engine,devoncarew/sky_engine,lyceel/engine,devoncarew/sky_engine,mxia/engine,qiankunshe/sky_engine,Hixie/sky_engine,zhangxq5012/sky_engine,chinmaygarde/flutter_engine,rmacnak-google/engine,afandria/sky_engine,Hixie/sky_engine,chinmaygarde/sky_engine,TribeMedia/sky_engine,axinging/sky_engine,abarth/sky_engine,rmacnak-google/engine,chinmaygarde/sky_engine,afandria/sky_engine,mpcomplete/engine,xunmengfeng/engine,iansf/sky_engine,jason-simmons/flutter_engine,cdotstout/sky_engine,Hixie/sky_engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,abarth/sky_engine,mpcomplete/flutter_engine,tvolkert/engine,xunmengfeng/engine,axinging/sky_engine,afandria/sky_engine,mikejurka/engine,xunmengfeng/engine,axinging/sky_engine,TribeMedia/sky_engine,jason-simmons/sky_engine,mikejurka/engine,devoncarew/sky_engine,mxia/engine,krisgiesing/sky_engine,jamesr/sky_engine,abarth/sky_engine,mxia/engine,mxia/engine,xunmengfeng/engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,lyceel/engine,aam/engine,mpcomplete/engine,mdakin/engine,mdakin/engine,aam/engine,axinging/sky_engine,mdakin/engine,cdotstout/sky_engine,axinging/sky_engine,flutter/engine,qiankunshe/sky_engine,jason-simmons/flutter_engine,devoncarew/engine,mdakin/engine,mikejurka/engine,jason-simmons/flutter_engine,mpcomplete/engine,rmacnak-google/engine,abarth/sky_engine,qiankunshe/sky_engine,xunmengfeng/engine,axinging/sky_engine,zhangxq5012/sky_engine,jamesr/flutter_engine,chinmaygarde/sky_engine,jamesr/flutter_engine,qiankunshe/sky_engine,afandria/sky_engine,aam/engine,tvolkert/engine,mxia/engine,TribeMedia/sky_engine,mdakin/engine,mpcomplete/engine,xunmengfeng/engine,chinmaygarde/flutter_engine,TribeMedia/sky_engine,zhangxq5012/sky_engine,jamesr/sky_engine,devoncarew/sky_engine,mpcomplete/engine,cdotstout/sky_engine,devoncarew/engine,zhangxq5012/sky_engine,mdakin/engine,jamesr/flutter_engine,Hixie/sky_engine,axinging/sky_engine,cdotstout/sky_engine,Hixie/sky_engine,lyceel/engine,Hixie/sky_engine,zhangxq5012/sky_engine,chinmaygarde/sky_engine,mpcomplete/flutter_engine,TribeMedia/sky_engine,mdakin/engine,devoncarew/sky_engine,iansf/sky_engine,mpcomplete/engine,afandria/sky_engine,krisgiesing/sky_engine,mxia/engine,qiankunshe/sky_engine,jason-simmons/flutter_engine,devoncarew/sky_engine,TribeMedia/sky_engine,jamesr/sky_engine,devoncarew/engine,zhangxq5012/sky_engine,afandria/sky_engine,mxia/engine,chinmaygarde/sky_engine,chinmaygarde/sky_engine,qiankunshe/sky_engine,TribeMedia/sky_engine,krisgiesing/sky_engine,Hixie/sky_engine,afandria/sky_engine,axinging/sky_engine,jamesr/sky_engine,jamesr/flutter_engine,zhangxq5012/sky_engine,iansf/sky_engine,zhangxq5012/sky_engine,mpcomplete/engine,mdakin/engine,qiankunshe/sky_engine,lyceel/engine,jason-simmons/sky_engine,TribeMedia/sky_engine,flutter/engine,lyceel/engine,jason-simmons/sky_engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,mpcomplete/engine,mxia/engine,cdotstout/sky_engine,krisgiesing/sky_engine,jamesr/flutter_engine