add zero check

Author: godzillaba

Diff for: src/bridge/ERC20Bridge.sol

@@ -65,6 +65,9 @@ contract ERC20Bridge is AbsBridge, IERC20Bridge {
     /// @notice When upgrading a custom fee chain from v1.x.x to v2.1.2, nativeTokenDecimals must be set to 18.
     ///         This is because v1.x.x contracts assume 18 decimals, but the ERC20Bridge does not have the decimals set in storage.
     function postUpgradeInit() external onlyDelegated onlyProxyOwner {
+        // this zero check might save you from accidentally upgrading from v2.x.x to v2.1.2
+        // it will not save you if your native token is supposed to have 0 decimals
+        require(nativeTokenDecimals == 0, "NONZERO_NATIVE_TOKEN_DECIMALS");
         nativeTokenDecimals = 18;
     }