libweston: move asserts to add_pending_output()

weston_compositor_add_pending_output() is the point through which all
backends must go when creating a new output. The enable and disable
vfuns are essential for anything to be done with the output, so it makes
sense to check them here, rather than when actually enabling or
disabling.

Particularly the disable vfunc is rarely called, so this gets the check
better excercised.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>