fix crash when missing dref in mp4 files
fix crash when missing dref in mp4 files

nguyen-viet-thanh-trung/gpac,aymanelyaagoubi/gpac,nguyen-viet-thanh-trung/gpac,porcelijn/gpac,rbouqueau/gpac,porcelijn/gpac,rauf/gpac,gpac/gpac,rbouqueau/gpac_brew_travis,rauf/gpac,nguyen-viet-thanh-trung/gpac,RodolpheFouquet/gpac,porcelijn/gpac,aymanelyaagoubi/gpac,gpac/gpac,rbouqueau/gpac_brew_travis,rauf/gpac,gpac/gpac,ARSekkat/gpac,nguyen-viet-thanh-trung/gpac,rauf/gpac,gpac/gpac,aymanelyaagoubi/gpac,ARSekkat/gpac,rbouqueau/gpac,emmanouil/gpac,rbouqueau/gpac,rbouqueau/gpac,rbouqueau/gpac,gpac/gpac,rauf/gpac,rbouqueau/gpac,ARSekkat/gpac,ARSekkat/gpac,rauf/gpac,porcelijn/gpac,ARSekkat/gpac,emmanouil/gpac,RodolpheFouquet/gpac,rbouqueau/gpac_brew_travis,porcelijn/gpac,gpac/gpac,nguyen-viet-thanh-trung/gpac,rbouqueau/gpac,aymanelyaagoubi/gpac,ARSekkat/gpac,emmanouil/gpac,rbouqueau/gpac_brew_travis,rbouqueau/gpac_brew_travis,emmanouil/gpac,RodolpheFouquet/gpac,RodolpheFouquet/gpac,aymanelyaagoubi/gpac,nguyen-viet-thanh-trung/gpac,rbouqueau/gpac,rbouqueau/gpac_brew_travis,emmanouil/gpac,RodolpheFouquet/gpac,porcelijn/gpac,gpac/gpac,emmanouil/gpac,aymanelyaagoubi/gpac,gpac/gpac,RodolpheFouquet/gpac