scanner: Fail on empty enumerations

Without this patch, the scanner would generate invalid C which wouldn't
compile anyway, so lets be nice and fail earlier and point out where the
error is.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: David Fort <contact@hardening-consulting.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>