simplecpp::TokenList::constFold does not fold '( 0 ) && 10 < X' properly

Test code:
```
    std::istringstream i("( 0 ) && 10 < X");
    simplecpp::TokenList tokenList(i, filenames);
    tokenList.constFold();
    tokenList.dump();
```
Output:
```
( 0 < X )
```

As `0 && expr` is 0 I believe the output should just be 0.
