ffmpeg: avoid NULL dereference with filters.

icodec can be NULL if there is no input stream
clearly associated with the output stream.

ffmpeg.c

@@ -3103,7 +3103,8 @@ static int transcode_init(void)
                     ost->filter->filter->inputs[0]->sample_aspect_ratio;
                 codec->pix_fmt = ost->filter->filter->inputs[0]->format;
 
-                if (codec->width   != icodec->width  ||
+                if (!icodec ||
+                    codec->width   != icodec->width  ||
                     codec->height  != icodec->height ||
                     codec->pix_fmt != icodec->pix_fmt) {
                     codec->bits_per_raw_sample = frame_bits_per_raw_sample;