Both are provided by libm, but `isnan()` is provided as a macro, whereas
`isfinite()` is an actual function, and hence libm has to be available
at runtime. That didn’t trivially work on FreeBSD, resulting in this
refactor.

`isfinite(x)` is equivalent to `!isnan(x) && !isinfinite(x)`. The case
of `x` being (negative or positive) infinity is already handled by the
range checks on the next line, so it’s safe to switch to `isnan()` here.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>