fix crash when quitting WebVTT playback after the end of the stream
fix crash when quitting WebVTT playback after the end of the stream

git-svn-id: ab66a9de07fa9d47c5829c82992f5279466c775f@4688 63c20433-aa62-49bd-875c-5a186b69a8fb

rbouqueau/gpac,RodolpheFouquet/gpac,nguyen-viet-thanh-trung/gpac,canatella/gpac,nguyen-viet-thanh-trung/gpac,nguyen-viet-thanh-trung/gpac,rbouqueau/gpac,aymanelyaagoubi/gpac,rauf/gpac,porcelijn/gpac,porcelijn/gpac,rbouqueau/gpac_brew_travis,Bevara/Access-open,drakeguan/gpac,nguyen-viet-thanh-trung/gpac,ARSekkat/gpac,emmanouil/gpac,RodolpheFouquet/gpac,vladimir-kazakov/gpac,RodolpheFouquet/gpac,DmitrySigaev/gpac,epam/gpac,DmitrySigaev/gpac,vladimir-kazakov/gpac,aymanelyaagoubi/gpac,rauf/gpac,rauf/gpac,Bevara/Access-open,rbouqueau/gpac_brew_travis,canatella/gpac,drakeguan/gpac,porcelijn/gpac,gpac/gpac,vladimir-kazakov/gpac,rbouqueau/gpac,aymanelyaagoubi/gpac,emmanouil/gpac,canatella/gpac,psteinb/gpac,aymanelyaagoubi/gpac,vladimir-kazakov/gpac,rbouqueau/gpac_brew_travis,ARSekkat/gpac,rbouqueau/gpac,canatella/gpac,ARSekkat/gpac,canatella/gpac,ARSekkat/gpac,emmanouil/gpac,emmanouil/gpac,rauf/gpac,Bevara/Access-open,Bevara/Access-open,drakeguan/gpac,psteinb/gpac,psteinb/gpac,RodolpheFouquet/gpac,gpac/gpac,epam/gpac,porcelijn/gpac,rauf/gpac,epam/gpac,rauf/gpac,aymanelyaagoubi/gpac,ARSekkat/gpac,gpac/gpac,Bevara/Access-open,nguyen-viet-thanh-trung/gpac,emmanouil/gpac,rbouqueau/gpac,ARSekkat/gpac,DmitrySigaev/gpac,psteinb/gpac,nguyen-viet-thanh-trung/gpac,emmanouil/gpac,canatella/gpac,drakeguan/gpac,vladimir-kazakov/gpac,DmitrySigaev/gpac,porcelijn/gpac,epam/gpac,gpac/gpac,canatella/gpac,rbouqueau/gpac,Bevara/Access-open,RodolpheFouquet/gpac,rbouqueau/gpac,rbouqueau/gpac_brew_travis,DmitrySigaev/gpac,drakeguan/gpac,DmitrySigaev/gpac,vladimir-kazakov/gpac,epam/gpac,rbouqueau/gpac_brew_travis,gpac/gpac,gpac/gpac,psteinb/gpac,aymanelyaagoubi/gpac,epam/gpac,RodolpheFouquet/gpac,drakeguan/gpac,psteinb/gpac,porcelijn/gpac,rbouqueau/gpac,rbouqueau/gpac_brew_travis,gpac/gpac,gpac/gpac