Add comment to memory.rs change

Author: birchmd

core/src/memory.rs

@@ -141,8 +141,12 @@ impl Memory {
 		len: U256,
 		data: &[u8],
 	) -> Result<(), ExitFatal> {
+		// Needed to pass ethereum test defined in
+		// https://github.com/ethereum/tests/commit/17f7e7a6c64bb878c1b6af9dc8371b46c133e46d
+		// (regardless of other inputs, a zero-length copy is defined to be a no-op).
+		// TODO: refactor `set` and `copy_large` (see
+		// https://github.com/rust-blockchain/evm/pull/40#discussion_r677180794)
 		if len.is_zero() {
-			// a zero-length copy is defined to be a no-op
 			return Ok(());
 		}