shell: check for missing wl_shell_surface

shell: check for missing wl_shell_surface

get_shell_surface() returns NULL, if the client has not created or has
destroyed the wl_shell_surface object.

All but one use of get_shell_surface() just retrieve the surface type,
so just fall back to SHELL_SURFACE_NORMAL there.

Resize hot-key binding really needs the wl_shell_surface object, as that
is the only way to send configure events. For surfaces without a
wl_shell_surface, simply do not resize them.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>