expect instead of unwrap

Co-authored-by: Philipp Gackstatter <PhilippGackstatter@users.noreply.github.com>

miden-crypto/src/merkle/sparse_path.rs

@@ -283,7 +283,7 @@ impl Iterator for SparseMerklePathIter<'_> {
         self.next_depth = this_depth.get() - 1;
 
         // `this_depth` is only ever decreasing, so it can't ever exceed `self.path.depth()`.
-        let node = self.path.at_depth(this_depth).unwrap();
+        let node = self.path.at_depth(this_depth).expect("current depth should never exceed the path depth");
         Some(node)
     }