sipaccount: fix nullptr access

SipAccount::CheckNATAddress may use regc_ member
when this one has been previsouly destroyed
and set to nullptr.
This patch checks regc_ value and take care to return correctly,
to prevent call to pjsip_regc_update_contact with nullptr regc.

Change-Id: I97590debd672095fed63ced40d3c541333b773f8
Tuleap: #172