Check if erc20 and zrc20 are token contracts in v2 contracts

[skosito]

          Here we're assuming token is an ERC20 compliant address always. To code this defensively we'd check always if it's a token contract.

Also we need to check if to != address(0).

    function isERC20(address token) internal view returns (bool) {
        return token.code.length > 0 && IERC20(token).totalSupply() > 0;
    }

    function withdrawAndRevert(address token, address to, uint256 amount, bytes calldata data) public nonReentrant {
        require(isERC20(token), "Provided address is not an ERC20 token");
        require(token != address(0), "Provided address is invalid");
        IERC20(token).safeTransfer(address(gateway), amount);

Originally posted by @fbac in #217 (comment)