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
On Thursday 24 July 2014 13:38:12 monarch dodra wrote:
> 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.

That would bring a slight change of behavior with it: All digits would always 
be popped. Whereas, at the moment, popping stops on overflow. Not sure if 
that's a problem.