Fix writing first audio Cues in dash mode.

In dahsmode Matroska is not writing the first Cluster for every
audio stream in the Cues element.

Signed-off-by: Frank Galligan <frankgalligan@gmail.com>
Reviewed-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>