Verify Bob's redeem encsig as Alice

Not doing so means that receiving an invalid encrypted signature from
Bob would make the generator produce a `RedeemBtc` action that should
not be accepted by the node (since Bob's signature would be invalid
after decrypting his encrypted signature).

It's better to fail early and let the user know what went wrong,
rather than let them hit an incomprehensible error when using their
wallet.

xmr-btc/src/alice.rs

@@ -99,6 +99,9 @@ where
     /// Reason why the swap has failed.
     #[derive(Debug)]
     enum Reason {
+        /// Bob's encrypted signature on the Bitcoin redeem transaction is
+        /// invalid.
+        InvalidEncryptedSignature,
         /// The refund timelock has been reached.
         BtcExpired,
     }
@@ -169,6 +172,17 @@ where
 
                 let tx_redeem = bitcoin::TxRedeem::new(&tx_lock, &redeem_address);
 
+                bitcoin::verify_encsig(
+                    B.clone(),
+                    s_a.into_secp256k1().into(),
+                    &tx_redeem.digest(),
+                    &tx_redeem_encsig,
+                )
+                .map_err(|_| SwapFailed::AfterXmrLock {
+                    reason: Reason::InvalidEncryptedSignature,
+                    tx_lock_height,
+                })?;
+
                 let sig_a = a.sign(tx_redeem.digest());
                 let sig_b =
                     adaptor.decrypt_signature(&s_a.into_secp256k1(), tx_redeem_encsig.clone());