sdp: Restore the original mp4 format h264 extradata if converted

If the sdp is generated before the rtp muxer is initialized
(e.g. as when called from the rtsp muxer), this has to be done,
otherwise the rtp muxer doesn't know that the input really is
in mp4 format.

Signed-off-by: Martin Storsjö <martin@martin.st>
