avformat/assenc: Use av_freep() to avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/assenc.c

@@ -84,7 +84,7 @@ static void purge_dialogues(AVFormatContext *s, int force)
         avio_printf(s->pb, "Dialogue: %s\r\n", dialogue->line);
         if (dialogue == ass->last_added_dialogue)
             ass->last_added_dialogue = next;
-        av_free(dialogue->line);
+        av_freep(&dialogue->line);
         av_free(dialogue);
         if (next)
             next->prev = NULL;