registrar(k): reset local contact holder

- when an UA fetches the bindings for a user and the user isn't
  registered, properly return an empty contact.
- closes SF#3035778, patch by Alex Hermann
(cherry picked from commit 776d941bef2e97089493a896bf0b57607078c52d)

modules_k/registrar/save.c

@@ -456,6 +456,8 @@ static inline int insert_contacts(struct sip_msg* _m, contact_t* _c,
 		if (r->contacts)
 			build_contact(r->contacts);
 		ul.release_urecord(r);
+	} else { /* No contacts found */
+		build_contact(NULL);
 	}
 
 #ifdef USE_TCP