avformat/webmdashenc: Check codec types

avformat/webmdashenc: Check codec types

The WebM DASH Manifest muxer only supports VP8, VP9, Vorbis and Opus,
but there was no check for this. The codec type is used to get a pointer
to a string containing the codec name or NULL if it is not one of those
four codecs. Said pointer has then been used without further checks as
string for the %s conversion specifier in an avio_printf()) call which
is undefined behaviour.

This commit adds a check for the supported codec types.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>