Cannot send UPDATE when call is ringing

[nanang]

Scenario:

1. A calls B
2. B send ringing/180 response
3. A changes IP address, so it shutdowns transport, unregisters and reregisters.
4. A tries to update its Contact header via UPDATE without SDP, but fails, log lines:

   pjsua_call.c  ....Sending UPDATE on call 0
   pjsua_call.c  .....Unable to send UPDATE because another media operation is in progress
   

In this case, i.e: there is pending SDP offer/answer, UPDATE without SDP should not be stopped as it won't change the media session.

Thanks Michael Ayles for the report.

[nanang]

In 5629:

Fix #2028: Cannot send UPDATE when call is ringing.