This is not strictly necessary, because if init fails, then
weston_compositor_backends_loaded() fails, main.c will
weston_compositor_destroy() -> weston_compositor_shutdown() ->
cmclcms_destroy() which will free this. But that is very hard to track
down, so let's make the code obviously more correct.

We must also avoid cmsDeleteContext(NULL), because it will then do
something to the default cms context rather than bail out.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>