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.

Author: TheBlueMatt

lightning/src/ln/chan_utils.rs

@@ -485,9 +485,11 @@ impl TxCreationKeys {
 }
 
 /// The maximum length of a script returned by get_revokeable_redeemscript.
-// Calculated as 6 bytes of opcodes, 1 byte push plus 2 bytes for contest_delay, and two public
-// keys of 33 bytes (+ 1 push).
-pub const REVOKEABLE_REDEEMSCRIPT_MAX_LENGTH: usize = 6 + 3 + 34*2;
+// Calculated as 6 bytes of opcodes, 1 byte push plus 3 bytes for contest_delay, and two public
+// keys of 33 bytes (+ 1 push). Generally, pushes are only 2 bytes (for values below 0x7fff, i.e.
+// around 7 months), however, a 7 month contest delay shouldn't result in being unable to reclaim
+// on-chain funds.
+pub const REVOKEABLE_REDEEMSCRIPT_MAX_LENGTH: usize = 6 + 4 + 34*2;
 
 /// A script either spendable by the revocation
 /// key or the broadcaster_delayed_payment_key and satisfying the relative-locktime OP_CSV constrain.