sipaccount: fix potential null dereference

Detected by sonarqube

Change-Id: I606f9cf2458dda07471d0a67af8915c7ca13d410

src/sip/sipaccount.cpp

@@ -789,6 +789,7 @@ SIPAccount::sendRegister()
     if (pjsip_regc_set_transport(regc, &tp_sel) != PJ_SUCCESS)
         throw VoipLinkException("Unable to set transport");
 
+    if (tp_sel.u.transport)
         setUpTransmissionData(tdata, tp_sel.u.transport->key.type);
 
     // pjsip_regc_send increment the transport ref count by one,