Two bugfixes:

1. the RED packet handling was incorrectly offsetting into final buffer by the total header length instead of the rtp header length.  in theory this matters a lot; in practice, it worked because the size of the red header doesn't seem to be different from the size of the RTP header (???)
2. Not accounting for RTP packet padding in the recorder; RTP packets can (again, in theory) be padded with stuff after the payload.  Strip that off before we hand it down to the rest of the RTP code so it doesn't have to deal with it.