unlock mutex when releasing object
unlock mutex when releasing object

Its locking the mutex at the start of func call, but when releasing a object by passing `nil`, there is no `Unlock()` before return. So whole program freeze after one `Export(nil,.........)` call.
aulanov/go.dbus,kyrofa/dbus,godbus/dbus,tpltnt/dbus,jonboulle/dbus