[zendframework/zendframework#4444] Fix usage of normalizeLibOptionKey

- normalizeLibOptionKey() uses pass-by-reference, but statement was
  assuming a return value.
- fixed method to use pass-by-reference

src/Storage/Adapter/MemcachedResourceManager.php

@@ -282,14 +282,14 @@ public function getLibOption($id, $key)
             throw new Exception\RuntimeException("No resource with id '{$id}'");
         }
 
-        $constValue = $this->normalizeLibOptionKey($key);
+        $this->normalizeLibOptionKey($key);
         $resource   = & $this->resources[$id];
 
         if ($resource instanceof MemcachedResource) {
-            return $resource->getOption($constValue);
+            return $resource->getOption($key);
         }
 
-        return isset($resource['lib_options'][$constValue]) ? $resource['lib_options'][$constValue] : null;
+        return isset($resource['lib_options'][$key]) ? $resource['lib_options'][$key] : null;
     }
 
     /**