ffmpeg_filter: check encoder before using it to set frame size.

Fix a segfault when no encoder is found for a default codec
with a complex filter graph.

ffmpeg_filter.c

@@ -1046,6 +1046,13 @@ int configure_filtergraph(FilterGraph *fg)
 
     for (i = 0; i < fg->nb_outputs; i++) {
         OutputStream *ost = fg->outputs[i]->ost;
+        if (!ost->enc) {
+            /* identical to the same check in ffmpeg.c, needed because
+               complex filter graphs are initialized earlier */
+            av_log(NULL, AV_LOG_ERROR, "Encoder (codec %s) not found for output stream #%d:%d\n",
+                     avcodec_get_name(ost->st->codec->codec_id), ost->file_index, ost->index);
+            return AVERROR(EINVAL);
+        }
         if (ost &&
             ost->enc->type == AVMEDIA_TYPE_AUDIO &&
             !(ost->enc->capabilities & AV_CODEC_CAP_VARIABLE_FRAME_SIZE))