Work around a Zig ReleaseSafe mode performance regression

With a little help from IRC:

<ifreund> Ayo: its probaly stupidly copying that array to the stack to do the
          safety check, pretty sure there's an open issue on this still
<ifreund> you may be able to work around the compiler's stupidity by using a
          pointer to the array or slice or something
<Ayo> ifreund: Yup, (&self.rdbuf)[self.rdoff] does the trick, thanks.
<ifreund> no problem! should get fixed eventually