chore: address code review comments of PR4398 (#4435)

# Description

## Problem\*

Related to #4170, this PR address comments in PR #4398 

## Summary\*

Add a comment and a sanity check.

## Additional Context



## Documentation\*

Check one:
- [X] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [X] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

acvm-repo/acvm/src/pwg/blackbox/hash.rs

@@ -143,12 +143,22 @@ pub(crate) fn solve_poseidon2_permutation_opcode(
         return Err(OpcodeResolutionError::BlackBoxFunctionFailed(
             acir::BlackBoxFunc::Poseidon2Permutation,
             format!(
-                "the number of inputs does not match specified length. {} > {}",
+                "the number of inputs does not match specified length. {} != {}",
                 inputs.len(),
                 len
             ),
         ));
     }
+    if len as usize != outputs.len() {
+        return Err(OpcodeResolutionError::BlackBoxFunctionFailed(
+            acir::BlackBoxFunc::Poseidon2Permutation,
+            format!(
+                "the number of outputs does not match specified length. {} != {}",
+                outputs.len(),
+                len
+            ),
+        ));
+    }
 
     // Read witness assignments
     let mut state = Vec::new();

noir_stdlib/src/hash/poseidon2.nr

@@ -93,7 +93,8 @@ impl Poseidon2 {
     }
 
     fn hash_internal<N>(input: [Field; N], in_len: u32, is_variable_length: bool) -> Field {
-        let iv : Field = (in_len as Field) * 18446744073709551616;
+        let two_pow_64 = 18446744073709551616;
+        let iv : Field = (in_len as Field) * two_pow_64;
         let mut sponge = Poseidon2::new(iv);
         for i in 0..input.len() {
             if i as u32 < in_len {