rpi: prevent segfault on shut-down due to NULL backend

When running with the RPi backend, Weston sefaults during shutdown. This is
due to the fact that `compositor->backend` is never initialised, and there
is no NULL check prior to calling `destroy()`.

This commit fixes the issue by making `rpi_backend_create()` intialise
`compositor->backend` - just like the X11 backend does.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91886

Signed-off-by: John Sadler <deathofathousandpapercuts@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor-rpi.c

@@ -531,6 +531,8 @@ rpi_backend_create(struct weston_compositor *compositor,
 		goto out_launcher;
 	}
 
+	compositor->backend = &backend->base;
+
 	return backend;
 
 out_launcher: