Watchtowers changed the code so that we *always* have a channel->shutdown_scriptpubkey[LOCAL]
(see new_channel()). The previous code had several problems:
1. It tested this for NULL, unnecessarily.
2. It allowed overriding if it was a default, *even* if we were already using it.
3. If the peer opened without option_shutdown_anysegwit, but upgraded before we closed,
we would not recognize the default.
4. It set the final scriptpubkey (and other things!) even if the command failed.
Changelog-Fixed: JSON-RPC: `close` with `destination` works even if prior `destination` was rejected.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
eb6e6bd373