avformat/dashdec: Reset pointer to NULL after freeing it

This is currently safe here, because the effective lifetime of
adaptionset_lang is parse_manifest_adaptationset() (i.e. the pointer
gets overwritten each time on entry to the function and gets freed
before exiting the function), but it is nevertheless safer to reset the
pointer.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 616eb93a40)

libavformat/dashdec.c

@@ -1156,6 +1156,7 @@ static int parse_manifest_adaptationset(AVFormatContext *s, const char *url,
 
 err:
     xmlFree(c->adaptionset_lang);
+    c->adaptionset_lang = NULL;
     return ret;
 }