Fix a crash due to wrong handling of 200 OK to REGISTER

When SIP un-registration request got an 200 OK with expiration value which is not 0 (due to the
server setting and network), it is treated as registration success based on the non zero expiration value. This made the other procedures following and it went into an uncontrollable
stage, i.e. unstoppable loop of the un-registration requests until crash.

Solution
Retrieve the requested expiration if possible, and check it together with the received expiration
in response to identify if the 200 OK is to register or unregister.