avformat/dashdec: Ensure strings are zero-terminated

avformat/dashdec: Ensure strings are zero-terminated

strncpy only ensures that one does not write beyond the end of the
destination buffer; in case of truncation it does not zero-terminate
the destination buffer. This makes using it the way it is now in the
DASH demuxer dangerous. So use av_strlcpy instead.

Also don't write anything if there is no id: The buffer has already been
zeroed initially.

The DASH testset from the Universität Klagenfurt contains samples with
ids that are too long. E.g.
http://ftp.itec.aau.at/datasets/DASHDataset2014/TearsOfSteel/1sec/TearsOfSteel_1s_simple_2014_05_09.mpd

Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>