Fix null pointer deref (crash) in new_user() when "client" object is invalid

When system.new_user() was called but the current "client" object is uninitialized (e.g. has NULL protocol, host or IP address fields because there is no active client, e.g. because is was called from a timed event with active user online) - this code would dereference a NULL pointer and crash the b0rad. Reported by Mortifis.