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.

Fixes: 1a4f87de
	"libweston: introduce weston_paint_node"

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