Set codec ID in the codec context by default.

gistfile1.diff

diff --git a/libavcodec/options.c b/libavcodec/options.c
index 2e41ce4..9e5f08a 100644
--- a/libavcodec/options.c
+++ b/libavcodec/options.c
@@ -84,6 +84,7 @@ int avcodec_get_context_defaults3(AVCodecContext *s, const AVCodec *codec)
     s->av_class = &av_codec_context_class;
 
     s->codec_type = codec ? codec->type : AVMEDIA_TYPE_UNKNOWN;
+    s->codec_id   = codec ? codec->id   : AV_CODEC_ID_NONE;
     s->codec      = codec;
     av_opt_set_defaults(s);