rtpbasepayload: Update current seqnum for buffer lists

The current sequence number will be the one from the first RTP buffer
when a buffer list is pushed, but should be the last one.

Fixes #495
2 jobs for master in 4 minutes and 15 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  Manifest
passed manifest #43899

00:00:20

 
  Build
passed build fedora x86_64 #43900

00:03:54