Allow existing locks to be cleared

Signed-off-by: Jim Zhang <jim.zhang@kaleido.io>

Author: jimthematrix

solidity/contracts/lib/zeto_common.sol

@@ -53,11 +53,7 @@ abstract contract ZetoCommon is OwnableUpgradeable {
                 lockedProofs[proofHash] == msg.sender,
             "Proof already locked by another party"
         );
-        if (delegate != address(0)) {
-            lockedProofs[proofHash] = delegate;
-        } else {
-            lockedProofs[proofHash] = msg.sender;
-        }
+        lockedProofs[proofHash] = delegate;
     }
 
     function sortInputsAndOutputs(

solidity/test/zkDvP.ts

@@ -226,9 +226,10 @@ describe("DvP flows between fungible and non-fungible tokens based on Zeto with
       const utxo1 = newUTXO(100, Alice);
       const proof = await zetoAnonTests.prepareProof(circuit1, provingKey1, Alice, [utxo1, ZERO_UTXO], [utxo1, ZERO_UTXO], [Alice, {}]);
 
-      await expect(zkPayment.connect(Alice.signer).lockProof(proof.encodedProof, "0x0000000000000000000000000000000000000000")).fulfilled;
+      await expect(zkPayment.connect(Alice.signer).lockProof(proof.encodedProof, await Alice.signer.getAddress())).fulfilled;
       await expect(zkPayment.connect(Bob.signer).lockProof(proof.encodedProof, await Bob.signer.getAddress())).rejectedWith("Proof already locked by another party");
       await expect(zkPayment.connect(Alice.signer).lockProof(proof.encodedProof, await Bob.signer.getAddress())).fulfilled;
+      await expect(zkPayment.connect(Bob.signer).lockProof(proof.encodedProof, "0x0000000000000000000000000000000000000000")).fulfilled;
     });
   });