Prevent accidentally locking native tokens in SignatureDrop (#315)

* Prevent possibly locking native tokens

* remove duplicate check

Author: nkrishang

contracts/signature-drop/SignatureDrop.sol

@@ -251,9 +251,9 @@ contract SignatureDrop is
         uint256 platformFees = (totalPrice * platformFeeBps) / MAX_BPS;
 
         if (_currency == CurrencyTransferLib.NATIVE_TOKEN) {
-            if (msg.value != totalPrice) {
-                revert("!Price");
-            }
+            require(msg.value == totalPrice, "!Price");
+        } else {
+            require(msg.value == 0, "!Value");
         }
 
         CurrencyTransferLib.transferCurrency(_currency, _msgSender(), platformFeeRecipient, platformFees);