Diesel's SQLite backend has possible UTF-8 corruption

Summary

Diesel uses the `sqlite3_value_text` function to receive strings from SQLite while deserializing query results. We misinterpreted the corresponding [SQLite](https://sqlite.org/c3ref/value_blob.html) documentation that this function always returns a UTF-8 encoded string values as `*const c_char`. Based on that we used `str::from_utf8_unchecked` to construct a Rust string slice without any additional UTF-8 checks in place. It turned out that this function doesn't always return correct UTF-8 strings. For field of the SQLite side storage type `BLOB` this pointer can contain arbitrary bytes, which makes the usage of `str::from_utf8_unchecked` unsound as this violates the safety contract of `str` to only contain valid UTF-8 encoded Strings.

## Mitigation

The preferred mitigation to the outlined problem is to update to a Diesel version 2.3.8 or newer, which includes fixes for the problem.

## Resolution

Diesel now correctly checks whether the provides byte buffer is actually valid UTF-8, instead of relying on SQLite's documentation. This fix is included in the `2.3.8` release.