fix: off by one error during log topic decoding

[mattsse]

Motivation

Fixes a bug, that is essentially an off by one error, that resulted in errors when decoding events with no indexed entries.

Solution

Remove the bad is_empty requirement and test against real log.

[prestwich]

can't this subtraction underflow if the LOG0 opcode is used? I'm not super familiar with these codepaths 🤔

[mattsse]

The macro generates different code for anonymous events:

ethers-rs/ethers-contract/ethers-contract-derive/src/lib.rs

Lines 411 to 440 in 94797f5

	let (signature_check, flat_topics_init, topic_tokens_len_check) = if event.anonymous {
	(
	quote! {},
	quote! {
	let flat_topics = topics.iter().flat_map(|t| t.as_ref().to_vec()).collect::<Vec<u8>>();
	},
	quote! {
	if topic_tokens.len() != topics.len() {
	return Err(ethers_core::abi::Error::InvalidData);
	}
	},
	)
	} else {
	(
	quote! {
	let event_signature = topics.get(0).ok_or(ethers_core::abi::Error::InvalidData)?;
	if event_signature != &Self::signature() {
	return Err(ethers_core::abi::Error::InvalidData);
	}
	},
	quote! {
	let flat_topics = topics.iter().skip(1).flat_map(|t| t.as_ref().to_vec()).collect::<Vec<u8>>();
	},
	quote! {
	if topic_tokens.len() != topics.len() - 1 {
	return Err(ethers_core::abi::Error::InvalidData);
	}
	},
	)
	};

only for non anonymous it makes sure that the signature is the first entry in topics

ethers-rs/ethers-contract/ethers-contract-derive/src/lib.rs

Line 426 in 94797f5

let event_signature = topics.get(0).ok_or(ethers_core::abi::Error::InvalidData)?;

so topics.len() - 1 can't underflow.

Most of this is adapted from:
https://github.com/rust-ethereum/ethabi/blob/970d47018e0f457440851752a7b46a4dbf793a0b/ethabi/src/event.rs#L122-L173