sipaccount: re-create transport if needed

After a connectivityChange() sometimes the TLS connection may fail
with a connection timeout. In this case, re-sending the register
without re-creating a transport will just use an unsuitable transport.
So, if the registration counter is 0, use doRegister() instead of
sendRegister() in order to reset the transports

Change-Id: I7ce09d8377fb3eee354a54cca8d793a641db5532
GitLab: #1224