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