color-device: Make sure lcms_context is not NULL

lcms interprets a NULL context as using a default, non-thread-safe
context, which is unsuitable for mutter's use. Make sure we're always
using a non-trivial context.

Helps: https://gitlab.gnome.org/GNOME/mutter/-/issues/2659


Signed-off-by: Simon McVittie <smcv@debian.org>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2877>


(cherry picked from commit e2be2d27)