lavf/webvttenc: Require webvtt file to contain exactly one WebVTT stream.

Not requiring this can end up producing hilariously broken files
together with -c:s copy (e.g. a webvtt file containing binary subtitle data).

Signed-off-by: Simon Thelen <ffmpeg-dev@c-14.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit b84232694e)

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

libavformat/webvttenc.c

@@ -46,8 +46,14 @@ static void webvtt_write_time(AVIOContext *pb, int64_t millisec)
 static int webvtt_write_header(AVFormatContext *ctx)
 {
     AVStream     *s = ctx->streams[0];
+    AVCodecContext *avctx = ctx->streams[0]->codec;
     AVIOContext *pb = ctx->pb;
 
+    if (ctx->nb_streams != 1 || avctx->codec_id != AV_CODEC_ID_WEBVTT) {
+        av_log(ctx, AV_LOG_ERROR, "Exactly one WebVTT stream is needed.\n");
+        return AVERROR(EINVAL);
+    }
+
     avpriv_set_pts_info(s, 64, 1, 1000);
 
     avio_printf(pb, "WEBVTT\n");