number can be overflow when use "**"

in the following contract, the field "number" can be overflow when use function setnumber. And in the report, it doesn't find this issue

```
contract HelloWorld {
string name;
uint8 number;

constructor() public {
name = "Hello, World!";
}

function get() public view returns (string memory) {
return name;
}

function set(string memory n) public {
name = n;
}

function setnumber() public {
number = number ** 22;
}
}
```