Do not core dump when destroying a non-fucntional "wlmessage"

If libwlmessage cannot run because a Wayland compositor has not been
found, and so the message window has not been completely initialized,
destroy it safely.

Change-Id: I86f897f9a5d96be0fc215c2bb311ae6bb158942d
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>