Fix panic in Postgres Bytes decode

[e00E]

This function can panic due to slicing out of bounds when the server
responds without the \x prefix. With this commit we instead error and
also ensure that the prefix is what we expect instead of blindly
removing it.

Not directly related to the panic, we replace as_str() with as_bytes()
because there is no reason to perform a utf8 validity check when
hex::decode already checks that the content is valid.

[abonander]

Good fix, though should we perhaps have a regression test?

It should be as simple as something like:

let mut conn = new::<Postgres>().await?;

conn.execute("SET bytea_output = 'escape';").await?;

let res: sqlx::Result<Vec<u8>> = conn.fetch_one("SELECT '\xDEADBEEF'::bytea")
    .await?
    .try_get(0usize);

// Binary deserialization only supports the `hex` format, but should not panic.
let _ = res.unwrap_err();

[e00E]

I've added a test. I had to adapt your example to test an empty hex string to trigger the panic without this commit.