SerialNumber::generate should be infallible

[str4d]

The method is currently fallible:

formats/x509-cert/src/serial_number.rs

Lines 73 to 82 in 48eac00

	impl<P: Profile> SerialNumber<P> {
	/// Generates a random serial number from RNG.
	///
	/// This follows the recommendation the CAB forum [ballot 164] and uses a minimum of 64 bits
	/// of output from the CSPRNG. This currently defaults to a 17-bytes long serial number.
	///
	/// [ballot 164]: https://cabforum.org/2016/03/31/ballot-164/
	pub fn generate(rng: &mut impl CryptoRngCore) -> Result<Self> {
	Self::generate_with_prefix(&[], 17, rng)
	}

However, it's a trivial wrapper around SerialNumber::generate_with_prefix with arguments that mean it cannot fail:

formats/x509-cert/src/serial_number.rs

Lines 98 to 105 in 48eac00

	// CABF requires a minimum of 64 bits of random
	if rand_len < 8 {
	return Err(ErrorKind::Failed.into());
	}
	if rand_len + prefix.len() > 19 {
	return Err(ErrorKind::Failed.into());
	}

formats/x509-cert/src/serial_number.rs

Lines 48 to 57 in 48eac00

	let inner = asn1::Uint::new(bytes)?;
	// The user might give us a 20 byte unsigned integer with a high MSB,
	// which we'd then encode with 21 octets to preserve the sign bit.
	// RFC 5280 is ambiguous about whether this is valid, so we limit
	// `SerialNumber` *encodings* to 20 bytes or fewer while permitting
	// `SerialNumber` *decodings* to have up to 21 bytes below.
	if inner.value_len()? > Self::MAX_LEN {
	return Err(ErrorKind::Overlength.into());
	}