Moving business logic preconditions to the beginning of the function.

contracts/token/BasicToken.sol

@@ -21,6 +21,7 @@ contract BasicToken is ERC20Basic {
   */
   function transfer(address _to, uint256 _value) public returns (bool) {
     require(_to != address(0));
+    require(_value <= balances[msg.sender]);
 
     // SafeMath.sub will throw if there is not enough balance.
     balances[msg.sender] = balances[msg.sender].sub(_value);