avconv: do not call avcodec_get_frame_defaults()

Strictly speaking it is not correct to call it on refcounted frames, in
any case it is unnecessary, since filtered_frame is always unreferenced
after poll_filter() returns.

avconv.c

@@ -642,8 +642,7 @@ static int poll_filter(OutputStream *ost)
 
     if (!ost->filtered_frame && !(ost->filtered_frame = av_frame_alloc())) {
         return AVERROR(ENOMEM);
-    } else
-        avcodec_get_frame_defaults(ost->filtered_frame);
+    }
     filtered_frame = ost->filtered_frame;
 
     if (ost->enc->type == AVMEDIA_TYPE_AUDIO &&