avformat/dashdec: Don't leave representation in inconsistent state on error

This currently doesn't cause any trouble, because the only caller did
not clean up the representation upon error at all; but fixing this is
a prerequisite for doing so.

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

libavformat/dashdec.c

@@ -627,7 +627,7 @@ static int parse_manifest_segmenturlnode(AVFormatContext *s, struct representati
                                                      initialization_val);
 
             if (!rep->init_section->url) {
-                av_free(rep->init_section);
+                av_freep(&rep->init_section);
                 xmlFree(initialization_val);
                 xmlFree(range_val);
                 return AVERROR(ENOMEM);