Fix invalid surface mapped signal

Prevents mapped signal of the surface without wayland buffer handle.
When a client became hidden just after it was shown, the surface was
sometimes mapped even though the handle of the back buffer was null.

Change-Id: Idc7d38cd27d0fe29525ba18163b4e43220fce125
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Reviewed-by: Elvis Lee <kwangwoong.lee@lge.com>