Groups GetGroupMembership count/list-length mismatch in EmberZNet v9.0.2

In EmberZNet v9.0.2 and earlier, malformed GetGroupMembership commands can trigger repeated reads past the end of the message payload and terminate the process. These messages must come from a device that has already joined the network, and no information leakage back to the sender was observed. Only devices supporting the Groups cluster may be impacted.