Without this commit it is possible that osmux is disabled again on links with
high jitter. This happens when an MGCP response without X-Osmux header is
received before the NAT receives an Osmux dummy frame from the other side.
Ticket: SYS#2628, SYS#2627
Sponsored-by: On-Waves ehf
Change-Id: Id624b0279aee5e2412059a10296ce7896e2d4628