Fix modexp record gas

[zjb0807]

frontier/frame/evm/precompile/modexp/src/lib.rs

Lines 176 to 187 in 272fe88

	if mod_len == 0 {
	return Ok(PrecompileOutput {
	exit_status: ExitSucceed::Returned,
	output: vec![],
	});
	}
	// Gas formula allows arbitrary large exp_len when base and modulus are empty, so we need to handle empty base first.
	let r = if base_len == 0 && mod_len == 0 {
	handle.record_cost(MIN_GAS_COST)?;
	BigUint::zero()
	} else {

Line 185 will not be executed because it will return at Line 176

[boundless-forest]

@nanocryk @notlesh Would you like to take a review about this?

[notlesh]

This case seems unclear to me, it's basically when bytes.len() > mod_len, which shouldn't be possible except for this case of mod_len == 0. Maybe this is better written by handling that case specifically rather than assume it is the case in the final catch-all } else { block.

Also see the comment above this (line ~ 215) which is relevant:

// always true except in the case of zero-length modulus, which leads to
// output of length and value 1.

This seems to contradict the expectation of returning an empty vec.