Check null CultureInfo when shrinking.

[Mono sometimes returns a null parent culture](https://github.com/mono/mono/blob/master/mcs/class/corlib/System.Globalization/CultureInfo.cs#L303)

[Microsoft always returns a non-null CultureInfo](http://referencesource.microsoft.com/#mscorlib/system/globalization/cultureinfo.cs#865)

https://github.com/fsharp/FsCheck/issues/37#issuecomment-46102440