avformat/dashdec: fix pointer being freed was not allocated

prevent attempt to call xmlFree if val was not allocated
fixes: 8135
Reviewed-by: Steven Liu <lq@onvideo.cn>
Signed-off-by: vectronic <hello.vectronic@gmail.com>

libavformat/dashdec.c

@@ -1203,6 +1203,7 @@ static int parse_programinformation(AVFormatContext *s, xmlNodePtr node)
         }
         node = xmlNextElementSibling(node);
         xmlFree(val);
+        val = NULL;
     }
     return 0;
 }