Fix panic when asking for a salted hash of key with len != 32

[gnunicorn]

[When upgrading substrate to latest [parity-db 2.0.0](https://github.com/paritytech/substrate/pull/8015), we [saw a panic in the benchmarking](https://gitlab.parity.io/parity/substrate/-/jobs/801118#L2252) of code we haven't touched. A short investigation revealed that the [recently added key salting feature](https://github.com/paritytech/parity-db/commit/64389057d3db2a72f92e43259db1a0ad65c66632) doesn't properly deal with keys of length other than 32`. This PR fixes that behavior for both circumstances of the key being smaller then (our case) but also larger than 32 character.

I wanted to add tests, but noticed there aren't any and that a general infrastructure for them is missing.. (e.g. easily create a testable Column)

@arkpar may I ask you to release this as 2.0.1 soon, so I can merge the dependencies PR?!?

[arkpar]

This only uses first 32 bytes of the key. If two long keys share the same 32 byte prefix there will be a collision. I've implemented a better fix in 8ba14c7 and published it as 0.2.1