lavf/ffmenc: do not fail on missing codec

ffm encoder fails when codec is not found.
It may happen when stream is being copied.
This commit allows to store such stream and provides
backward compatibility with version prior 2.5 release.

fixes #4266
Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>

libavformat/ffmenc.c

@@ -94,15 +94,18 @@ static void write_header_chunk(AVIOContext *pb, AVIOContext *dpb, unsigned id)
     av_free(dyn_buf);
 }
 
-static int ffm_write_header_codec_private_ctx(AVIOContext *pb, AVCodecContext *ctx, int type)
+static int ffm_write_header_codec_private_ctx(AVFormatContext *s, AVCodecContext *ctx, int type)
 {
+    AVIOContext *pb = s->pb;
     AVIOContext *tmp;
     char *buf = NULL;
     int ret;
     const AVCodec *enc = ctx->codec ? ctx->codec : avcodec_find_encoder(ctx->codec_id);
 
-    if (!enc)
-        return AVERROR(EINVAL);
+    if (!enc) {
+        av_log(s, AV_LOG_WARNING, "Stream codec is not found. Codec private options are not stored.\n");
+        return 0;
+    }
     if (ctx->priv_data && enc->priv_class && enc->priv_data_size) {
         if ((ret = av_opt_serialize(ctx->priv_data, AV_OPT_FLAG_ENCODING_PARAM | type,
                                     AV_OPT_SERIALIZE_SKIP_DEFAULTS, &buf, '=', ',')) < 0)
@@ -281,7 +284,7 @@ static int ffm_write_header(AVFormatContext *s)
                                                         st->recommended_encoder_configuration)) < 0)
                 return ret;
             } else if ((ret = ffm_write_header_codec_ctx(s->pb, codec, MKBETAG('S', '2', 'V', 'I'), AV_OPT_FLAG_VIDEO_PARAM)) < 0 ||
-                       (ret = ffm_write_header_codec_private_ctx(s->pb, codec, AV_OPT_FLAG_VIDEO_PARAM)) < 0)
+                       (ret = ffm_write_header_codec_private_ctx(s, codec, AV_OPT_FLAG_VIDEO_PARAM)) < 0)
                 return ret;
             break;
         case AVMEDIA_TYPE_AUDIO:
@@ -292,7 +295,7 @@ static int ffm_write_header(AVFormatContext *s)
                                                         st->recommended_encoder_configuration)) < 0)
                 return ret;
             } else if ((ret = ffm_write_header_codec_ctx(s->pb, codec, MKBETAG('S', '2', 'A', 'U'), AV_OPT_FLAG_AUDIO_PARAM)) < 0 ||
-                     (ret = ffm_write_header_codec_private_ctx(s->pb, codec, AV_OPT_FLAG_AUDIO_PARAM)) < 0)
+                     (ret = ffm_write_header_codec_private_ctx(s, codec, AV_OPT_FLAG_AUDIO_PARAM)) < 0)
                 return ret;
             break;
         default: