Prevent SIP.js stack to crash app on re-register when there is already an outstanding REGISTER request

[adaxi]

In our code base we took the approach of using the full API framework instead of using the SimpleUser class.

We instantiate the Registerer to register our user agent.

In some cases we are getting this uncaught error by the SIP.js stack causing our nodejs process to stop.

REGISTER request already in progress, waiting for final response

From my analysis this can happen if we attempt to call the register function, and in parallel the re-register function is called on timeout.

I have opted to check if the waiting flag is set instead of blindly catching the exception, but I am open to other suggestions.