Fix a crash for : flexisip_cli REGISTRAR_DELETE
Fix a crash for : flexisip_cli REGISTRAR_DELETE

It appears that sip->sip_contact can be NULL while using
REGISTRAR_DELETE flexisip_cli command.

(cherry picked from commit 7734e60c45701cd131530bc368b75299c779cf2f)

BelledonneCommunications/flexisip,BelledonneCommunications/flexisip,BelledonneCommunications/flexisip,BelledonneCommunications/flexisip