sealable-tire: verify expected error when hashing Extension nodes

Addresses a left-over `XXX` comment which slipped in previous commit.
Extension nodes never have empty keys.  Panic if that’s not held when
calculating node’s hash.

This mirrors code in SetContext::handle_extension where the opposite
invariant is held and code panics if the key is too long.

sealable-trie/src/nodes.rs

@@ -208,12 +208,16 @@ impl<'a, P, S> Node<'a, P, S> {
                 // tag = 0b100v_0000 where v indicates whether the child is
                 // a value reference.
                 let tag = 0x80 | (u8::from(child.is_value()) << 4);
-                // XXX
-                if let Ok(len) = key.encode_into(key_buf, tag) {
-                    buf[len..len + 32].copy_from_slice(child.hash().as_slice());
-                    len + 32
-                } else {
-                    return hash_extension_slow_path(*key, child);
+                match key.encode_into(key_buf, tag) {
+                    Ok(len) => {
+                        buf[len..len + 32]
+                            .copy_from_slice(child.hash().as_slice());
+                        len + 32
+                    }
+                    Err(bits::EncodeError::EmptySlice) => {
+                        return hash_extension_slow_path(*key, child);
+                    }
+                    Err(bits::EncodeError::SliceTooLong) => unreachable!(),
                 }
             }
         };