Affected: Hackage: scientific

# Out-of-bounds read in Eq and Ord on numbers with extreme exponents

`scientific-0.3.9.0` reimplemented `Eq` and `Ord` without normalising their arguments. The new implementation short-circuits on `integerLog10' c + e` computed in `Int`, which overflows when the base-10 exponent is close to `maxBound :: Int`. The overflowed comparison then falls through to `magnitude (e1 - e2)`, where the exponent difference overflows to a negative `Int` as well. `magnitude` indexed its cached power-of-ten array without a bounds check, so the negative index performed an out-of-bounds read and the process died with `SIGSEGV`.

With `scientific-0.3.9.0` (GHC 9.10.3, x86_64-linux), as reported in [basvandijk/scientific#120](https://github.com/basvandijk/scientific/issues/120):

```haskell import Data.Scientific

main :: IO () main = print (scientific 10 maxBound == scientific 1 minBound) ```

``` $ ./repro; echo "exit=$?" exit=139 ```

The issue was discovered when Stackage nightly test suites (`genvalidity-hspec-aeson`, `genvalidity-sydtest-aeson`) started segfaulting after upgrading to `scientific-0.3.9.0`: their property tests round-trip `aeson` `Value`s, which produces `Scientific`s with extreme exponents.

Versions before `0.3.9.0` did not crash, but `Eq`/`Ord` returned invalid results there instead (the normalisation-based implementation overflows the exponent, reporting `scientific 10 maxBound == scientific 1 minBound` as `True`). The same unchecked read was also reachable through `(+)`…