window: Fix crash when unexporting unrealized window

It is possible that the window gets unrealized while a handle to its
surface is exported. This, naturally, invalidates the handle, so there
is nothing left to unexport. We should just note that and do nothing,
rather than crashing.

Fixes a crash when a portal-backed file dialog parented to a window is
closed after the window it was parented to.

Reported-by: Ivan Molodetskikh <yalterz@gmail.com>
Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>