add a missing initialization of the m_memoryCache in XmlRpcGroupsServicesConnectorModule

the lack of this caused me a NullReferenceException when calling some groups methods directly though in principle it would also fail in other situations

OpenSim/Region/OptionalModules/Avatar/XmlRpcGroups/XmlRpcGroupsServicesConnectorModule.cs

@@ -142,8 +142,8 @@ namespace OpenSim.Region.OptionalModules.Avatar.XmlRpcGroups
                     m_log.InfoFormat("[XMLRPC-GROUPS-CONNECTOR]: Groups Cache Timeout set to {0}.", m_cacheTimeout);
                 }
 
-
                 // If we got all the config options we need, lets start'er'up
+                m_memoryCache = new ExpiringCache<string, XmlRpcResponse>();
                 m_connectorEnabled = true;
             }
         }