libweston: fix crash on never used output's tear-down

weston_output_enable() initializes the list, but weston_output_release()
maybe be called even if the output was never enabled, triggering the
assert due to uninitialized (actually NULL) list head.

This can be triggered with a bad weston.ini, for example using an
invalid output transform value.

Check in weston_output_disable() instead, but because it too may be
called for non-enabled output, only if it was actually enabled.

Fixes: 1a4f87de
	"libweston: introduce weston_paint_node"

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