fix issue 13163 - std.conv.parse misses overflow when it doesn't result in a smaller value

https://issues.dlang.org/show_bug.cgi?id=13163
If I remember correctly, `overflow` is "sticky", meaning once it is set to true, it stays that way. It might be better to declare it outside your loop, do all your math, and then check it only once when you are finished. Right, @WalterBright ?