rtp: fix up handling of extension header

[derkolja]

Hi,

I ran into a bit of a problem trying to use the extension header. I am reasonably confident this fix is correct for the one byte header case, however, the RFC also specifies a Two-Byte Header, which I'm not certain it is supported here at all.
https://datatracker.ietf.org/doc/html/rfc5285#section-4.2

I hope this helps, I'm afraid I don't have any experience contributing to open source projects.

:-) Kolja

• When setting the len field of the header, a bad offset of the source
  pointer was used
• The len field of the header is a uint16 specifying payload length in
  multiples of 32 bit
• Also, subtract the extension header length from the payload length

[jrsnen]

Hi!

Thank you for this PR. I don't see any problems with it, and I'm just going to merge it as is. The extension headers don't seem to be used in uvgRTP at the moment, but it is good to fix bugs in them regardless.

uvgRTP is still very much in its infancy, so there are lots of small things that need improving and fixing. Thank you for helping in this endeavor.