libweston-desktop: Fix weston crash when lost the shsurf

The shell_surface may disappear when keyboard lost focus,
then the shsurf will be NULL.
Have an ahead check for shsurf before calling the callback
in weston_desktop_surface_foreach_child.

Fixes #811

Tested-by: Erkai Ji <erkai.ji@nxp.com>
Signed-off-by: Wujian Sun <wujian.sun_1@nxp.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>