spl: small opt fn set_authority

[nkysg]

Rust’s Option type already supports as_ref for converting Option to Option<&Pubkey>

   pub const fn as_ref(&self) -> Option<&T> {
        match *self {
            Some(ref x) => Some(x),
            None => None,
        }
    }

Add explicit validation of the data length before conversion

pub const fn [from_le_bytes](https://doc.rust-lang.org/std/primitive.u64.html#method.from_le_bytes)(bytes: [[u8](https://doc.rust-lang.org/std/primitive.u8.html); [8](https://doc.rust-lang.org/std/primitive.array.html)]) -> [u64](https://doc.rust-lang.org/std/primitive.u64.html)

[Aursen]

After it seems good to me