[COLLECTIONS-441] throw an internal error when clone failed instead of returning null, which is against the contract of clone, also fixes a findbug error.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1443606 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/collections/map/AbstractHashedMap.java

@@ -1278,6 +1278,7 @@ public class AbstractHashedMap<K, V> extends AbstractMap<K, V> implements Iterab
      * <code>Cloneable</code> interface and make this method public.
      *
      * @return a shallow clone
+     * @throws java.lang.InternalError if {@link super#clone()} failed
      */
     @Override
     @SuppressWarnings("unchecked")
@@ -1294,7 +1295,7 @@ public class AbstractHashedMap<K, V> extends AbstractMap<K, V> implements Iterab
             cloned.putAll(this);
             return cloned;
         } catch (final CloneNotSupportedException ex) {
-            return null;  // should never happen
+            throw new InternalError();
         }
     }