Test failure when compiled with -O0

If I compile with `-O0`, I get the following test failure:

```
*** Failed! Falsifiable (after 1 test):
0.0
0.0
(used seed -8074436915145524066)
t_shortest: [Failed]
*** Failed! Falsifiable (after 1 test):
0.0
0.0
(used seed 5824167789149766384)
Regressions:
toShortest_overflow: [OK]

Properties Test Cases Total
Passed 0 1 1
Failed 2 0 2
Total 2 1 3
```

The test code in question is https://github.com/bos/double-conversion/blob/e4b7956d51fbb79ada95412519bd2a6a680def23/tests/Properties.hs#L9-L13

where `a = 0.0`, `b = 0.0`.

The problem seems to be the [use of](https://github.com/bos/double-conversion/blob/e4b7956d51fbb79ada95412519bd2a6a680def23/Data/Double/Conversion/ByteString.hs#L72) `realToFrac (0.0 / 0.0)`. Note that `0.0/0.0` is `NaN`.

* With `-O`, `realToFrac NaN` returns `NaN`
* With `-O0`, `realToFrac NaN` returns `-Infinity`

You would expect `realToFrac :: Double -> Double` to always be `id`, but apparently that is not the case in GHC right now. It is so only if compile with optimisations.