The original clamping bounds on `_x` actually produce finite values:
```
exp(88.3762626647950) = 2.40614e+38 < 3.40282e+38
exp(709.437) = 1.27226e+308 < 1.79769e+308
```
so with an accurate `ldexp` implementation, `pexp` fails for large
inputs, producing finite values instead of `inf`.
This adjusts the bounds slightly outside the finite range so that
the output will overflow to +/- `inf` as expected.
9fde9cce5d