ffmpeg_opt: check that a subtitle encoder is available before auto mapping streams

Fixes Ticket3470
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg_opt.c

@@ -1833,7 +1833,7 @@ static int open_output_file(OptionsContext *o, const char *filename)
 
         /* subtitles: pick first */
         MATCH_PER_TYPE_OPT(codec_names, str, subtitle_codec_name, oc, "s");
-        if (!o->subtitle_disable && (oc->oformat->subtitle_codec != AV_CODEC_ID_NONE || subtitle_codec_name)) {
+        if (!o->subtitle_disable && (avcodec_find_encoder(oc->oformat->subtitle_codec) || subtitle_codec_name)) {
             for (i = 0; i < nb_input_streams; i++)
                 if (input_streams[i]->st->codec->codec_type == AVMEDIA_TYPE_SUBTITLE) {
                     new_subtitle_stream(o, oc, i);