store allows constructing Ratio with zero denominator

[k0001]

Rational numbers can't contain a zero denominator. Smart constructors such as Data.Ratio.(%) prevent this, but store allows it.

Data.Store.decodeEx "\NUL\001\NUL\NUL\NUL\NUL\NUL\NUL\NUL\NUL\000\NUL\NUL\NUL\NUL\NUL\NUL\NUL" :: Rational 
1 % 0

> Data.Store.decodeEx "\NUL\000\NUL\NUL\NUL\NUL\NUL\NUL\NUL\NUL\000\NUL\NUL\NUL\NUL\NUL\NUL\NUL" :: Rational 
0 % 0

> Data.Store.decodeEx "\001\NUL\NUL\NUL\000\NUL\NUL\NUL" :: Ratio Int32
1 % 0

> Data.Store.decodeEx "\000\NUL\NUL\NUL\000\NUL\NUL\NUL" :: Ratio Int32
0 % 0

For comparison, other serialization/deserialization libraries take care of preventing a zero denominator (see k0001/safe-money#21)

Other than showing it, there's no much you can do with a Ratio with a zero denominator, since most operations on them fail. For example:

> 1 + (Data.Store.decodeEx "\000\NUL\NUL\NUL\000\NUL\NUL\NUL" :: Ratio Int32)
*** Exception: Ratio has zero denominator

store should always fail to parse Ratios that have a zero denominator.