Fix `REVOKEABLE_REDEEMSCRIPT_MAX_LENGTH` for contest delays >0x7fff

Fix `REVOKEABLE_REDEEMSCRIPT_MAX_LENGTH` for contest delays >0x7fff

When contest delays are >= 0x8000, script pushes require an extra
byte to avoid being interpreted as a negative int. Thus, for
channels with CSV delays longer than ~7.5 months we may generate
transactions with slightly too little fee. This isn't really a huge
deal, but we should prefer to be conservative here, and slightly
too high fee in the general case is better than slightly too little
fee in other cases.