Fix isom_write_stss.
Fix isom_write_stss.

If the sync sample box is not present, every sample is a random access point.

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