Remote denial of service in HyperLedger Fabric

This issue was caused by [a missing check of nil](https://github.com/hyperledger/fabric/pull/3494). > An orderer to orderer consensus message that contains an empty inner message crashes the node because it attempts to figure out its type and the mere action of determining the type of a nil pointer, causes a panic. Thank you to Haosheng Wang of OPPO ZIWU Security Lab for this disclosure.