ffmpeg: Check avcodec_parameters_to_context() for failure

Fixes CID1396241
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffmpeg.c

@@ -2877,7 +2877,8 @@ static int init_output_stream_streamcopy(OutputStream *ost)
 
     av_assert0(ist && !ost->filter);
 
-    avcodec_parameters_to_context(ost->enc_ctx, ist->st->codecpar);
+    ret = avcodec_parameters_to_context(ost->enc_ctx, ist->st->codecpar);
+    if (ret >= 0)
         ret = av_opt_set_dict(ost->enc_ctx, &ost->encoder_opts);
     if (ret < 0) {
         av_log(NULL, AV_LOG_FATAL,