common: Set AV_CODEC_ID_NONE to AVCodecContext.codec_id before avcodec_open2().

AVCodecContext.codec_id is supposed to be set properly in avcodec_open2().
This avoids avcodec_open2() failure by the difference of enum AVCodecID.
For instance, when stream is encoded as AC-3,
AVCodecContext.codec_id might have been set to AV_CODEC_ID_EAC3
while AVCodec.id is set to AV_CODEC_ID_AC3.