fix(maci): amended the signUp function to reflect the checks-effect…

…s-interaction pattern

Moved the `_pubKey` check above before increasing the number of signups.

fix #503

contracts/contracts/MACI.sol

@@ -199,18 +199,18 @@ contract MACI is IMACI, DomainObjs, Params, SnarkCommon, Ownable {
             "MACI: maximum number of signups reached"
         );
 
+        require(
+            _pubKey.x < SNARK_SCALAR_FIELD && _pubKey.y < SNARK_SCALAR_FIELD,
+            "MACI: _pubKey values should be less than the snark scalar field"
+        );
+
         // Increment the number of signups
         numSignUps++;
 
         // Register the user via the sign-up gatekeeper. This function should
         // throw if the user has already registered or if ineligible to do so.
         signUpGatekeeper.register(msg.sender, _signUpGatekeeperData);
 
-        require(
-            _pubKey.x < SNARK_SCALAR_FIELD && _pubKey.y < SNARK_SCALAR_FIELD,
-            "MACI: _pubKey values should be less than the snark scalar field"
-        );
-
         // Get the user's voice credit balance.
         uint256 voiceCreditBalance = initialVoiceCreditProxy.getVoiceCredits(
             msg.sender,