Fix for #400: Check that destination of token transfers is not 0x

contracts/token/BasicToken.sol

@@ -20,6 +20,8 @@ contract BasicToken is ERC20Basic {
   * @param _value The amount to be transferred.
   */
   function transfer(address _to, uint256 _value) returns (bool) {
+    require(_to != address(0));
+
     balances[msg.sender] = balances[msg.sender].sub(_value);
     balances[_to] = balances[_to].add(_value);
     Transfer(msg.sender, _to, _value);

contracts/token/StandardToken.sol

@@ -24,6 +24,8 @@ contract StandardToken is ERC20, BasicToken {
    * @param _value uint256 the amount of tokens to be transferred
    */
   function transferFrom(address _from, address _to, uint256 _value) returns (bool) {
+    require(_to != address(0));
+
     var _allowance = allowed[_from][msg.sender];
 
     // Check is not needed because sub(_allowance, _value) will already throw if this condition is not met