color-noop: avoid assert hit in cmnoop_destroy()

When we are destroying the color manager, the components referencing
color profiles should have already been destroyed. We have an assert
in cmnoop_destroy() to make sure that the stock profile has refcount
equal to 1.

But we currently have an issue in Weston. While shutting down with
client surfaces alive, we may leak them. So we try to destroy
the color manager with surfaces still alive, and they may be
referencing color profiles.

We already have a workaround for this in our LittleCMS color plugin,
but we've missed that in color-noop. This fixes that, so now we don't
hit the assert anymore.

As we are already dealing with asserts in color-noop, I took the
liberty to replace the last usage of assert with our own wrapper
in this file.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>