fragment: Fix writing track fragments when 'tfdt' box is absent.
fragment: Fix writing track fragments when 'tfdt' box is absent.

silverfilain/L-SMASH,silverfilain/L-SMASH,canbal/l-smash,dwbuiten/l-smash,maki-rxrz/L-SMASH,dwbuiten/l-smash,silverfilain/L-SMASH,canbal/l-smash,l-smash/l-smash,l-smash/l-smash,mstorsjo/l-smash,l-smash/l-smash,mstorsjo/l-smash,maki-rxrz/L-SMASH