avformat/dashdec: Fix leak of representation languages

These languages are normally freed after having been added as metadata
to their respective AVStreams. Yet if one never reaches said point, they
leak. This can happen as a result of an error when reading the header or
as a result of refreshing the manifests.

Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit f1c3c173c9)

libavformat/dashdec.c

@@ -359,6 +359,7 @@ static void free_representation(struct representation *pls)
     }
 
     av_freep(&pls->url_template);
+    av_freep(&pls->lang);
     av_freep(&pls);
 }