lavf/matroskadec: Ensure cues_end is initialized

Ensure that cues_start and cues_end are always initialized and
the webm_dash_manifest_cues function returns appropriate error if
they are not computed correctly.

This fixes coverity defect CID1231991.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>