If an SDP UPDATE is received after the 183, but before the 200,
SEMS fails when it receive the final ACK. In fact SEMS tries to
find the SDP content inside the ACK message itself.
The solution is copied from sems-pbx module where the issue
doesn't happen.
(real ticket number: TT#43503)
Change-Id: I5a432dc57c701d7eb0d5306d6005508e3310e7ba