libtxt: move to the next run if the current run ends before the start of the line block (#4891)
libtxt: move to the next run if the current run ends before the start of the line block (#4891)

Fixes https://github.com/flutter/flutter/issues/15975
jamesr/flutter_engine,aam/engine,chinmaygarde/sky_engine,mikejurka/engine,cdotstout/sky_engine,chinmaygarde/flutter_engine,jamesr/flutter_engine,mikejurka/engine,rmacnak-google/engine,rmacnak-google/engine,devoncarew/engine,cdotstout/sky_engine,Hixie/sky_engine,chinmaygarde/sky_engine,krisgiesing/sky_engine,jamesr/sky_engine,Hixie/sky_engine,tvolkert/engine,devoncarew/sky_engine,jason-simmons/sky_engine,devoncarew/sky_engine,jamesr/flutter_engine,tvolkert/engine,jason-simmons/sky_engine,jamesr/flutter_engine,devoncarew/sky_engine,flutter/engine,jason-simmons/flutter_engine,devoncarew/engine,jamesr/sky_engine,flutter/engine,jason-simmons/sky_engine,jamesr/sky_engine,cdotstout/sky_engine,aam/engine,flutter/engine,cdotstout/sky_engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,krisgiesing/sky_engine,aam/engine,devoncarew/sky_engine,rmacnak-google/engine,jason-simmons/sky_engine,tvolkert/engine,jamesr/sky_engine,mikejurka/engine,aam/engine,tvolkert/engine,chinmaygarde/sky_engine,devoncarew/sky_engine,tvolkert/engine,chinmaygarde/flutter_engine,Hixie/sky_engine,jamesr/flutter_engine,aam/engine,aam/engine,jamesr/flutter_engine,jason-simmons/flutter_engine,devoncarew/engine,Hixie/sky_engine,chinmaygarde/flutter_engine,mikejurka/engine,mikejurka/engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,rmacnak-google/engine,krisgiesing/sky_engine,jamesr/flutter_engine,rmacnak-google/engine,jason-simmons/flutter_engine,devoncarew/engine,tvolkert/engine,krisgiesing/sky_engine,jamesr/flutter_engine,jamesr/sky_engine,Hixie/sky_engine,krisgiesing/sky_engine,devoncarew/sky_engine,Hixie/sky_engine,aam/engine,flutter/engine,jason-simmons/flutter_engine,chinmaygarde/sky_engine,cdotstout/sky_engine,chinmaygarde/flutter_engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,mikejurka/engine,devoncarew/engine,aam/engine,jason-simmons/flutter_engine,chinmaygarde/sky_engine,jamesr/sky_engine,rmacnak-google/engine,rmacnak-google/engine,mikejurka/engine,jamesr/flutter_engine,tvolkert/engine,krisgiesing/sky_engine,krisgiesing/sky_engine,Hixie/sky_engine,devoncarew/engine,jamesr/sky_engine,flutter/engine,chinmaygarde/flutter_engine,mikejurka/engine,flutter/engine,cdotstout/sky_engine,devoncarew/engine,jason-simmons/flutter_engine,Hixie/sky_engine,chinmaygarde/flutter_engine,jason-simmons/sky_engine,jason-simmons/sky_engine,cdotstout/sky_engine,mikejurka/engine,chinmaygarde/sky_engine,flutter/engine,flutter/engine