Slight performance improvement to IntObjectHashMap.hashIndex()

Motivation:

Using a needless local copy of keys.length.

Modifications:

Using keys.length explicitly everywhere.

Result:

Slight performance improvement of hashIndex.

common/src/main/java/io/netty/util/collection/IntObjectHashMap.java

@@ -311,9 +311,8 @@ public class IntObjectHashMap<V> implements IntObjectMap<V>, Iterable<IntObjectM
      * Returns the hashed index for the given key.
      */
     private int hashIndex(int key) {
-        int length = keys.length;
         // Allowing for negative keys by adding the length after the first mod operation.
-        return (key % length + length) % length;
+        return (key % keys.length + keys.length) % keys.length;
     }
 
     /**