Add explicit throw to BasicToken.transfer

contracts/token/BasicToken.sol

@@ -30,7 +30,9 @@ contract BasicToken is ERC20Basic {
   * @param _value The amount to be transferred.
   */
   function transfer(address _to, uint256 _value) onlyPayloadSize(2 * 32) {
-    balances[msg.sender] = balances[msg.sender].sub(_value);
+    if(balances[msg.sender] < _value) throw;
+
+    balances[msg.sender] -= _value;
     balances[_to] = balances[_to].add(_value);
     Transfer(msg.sender, _to, _value);
   }