remuxer: Fix absent of 'sidx' within files branded with 'dash'.
remuxer: Fix absent of 'sidx' within files branded with 'dash'.

This is regression at 4ec6738 (support of multiple segments).

silverfilain/L-SMASH,l-smash/l-smash,l-smash/l-smash,l-smash/l-smash,maki-rxrz/L-SMASH,maki-rxrz/L-SMASH,silverfilain/L-SMASH,silverfilain/L-SMASH