vulkan/wsi/wayland: Initialise wl_shm pointer in VkImage

vulkan/wsi/wayland: Initialise wl_shm pointer in VkImage

We don't explicitly calloc the wsi_wl_image to zero; anything that's
expected to be valid needs to be explicitly initialised.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5063
Fixes: 6b36f35734a9 ("vulkan/wsi/wl: add wl_shm support for lavapipe.")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Tested-by: Jan Beich <jbeich@freebsd.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11909>