[FAB-5339] Add missing nil check to extensions.go

There is a missing nil check, that might make the code
result in a null pointer panic.

For reference: IsDataMsg is:

func (m *GossipMessage) IsDataMsg() bool {
	return m.GetDataMsg() != nil
}

Change-Id: I5e20dee188e863b932312a7963f0ca73965c1457
Signed-off-by: yacovm <yacovm@il.ibm.com>

protos/gossip/extensions.go

@@ -521,7 +521,7 @@ func (m *SignedGossipMessage) String() string {
 		var isSimpleMsg bool
 		if m.GetStateResponse() != nil {
 			gMsg = fmt.Sprintf("StateResponse with %d items", len(m.GetStateResponse().Payloads))
-		} else if m.IsDataMsg() {
+		} else if m.IsDataMsg() && m.GetDataMsg().Payload != nil {
 			gMsg = m.GetDataMsg().Payload.toString()
 		} else if m.IsDataUpdate() {
 			update := m.GetDataUpdate()

protos/gossip/extensions_test.go

@@ -72,6 +72,10 @@ func TestToString(t *testing.T) {
 		},
 	}
 	assert.NotContains(t, fmt.Sprintf("%v", sMsg), "2")
+	sMsg.GetDataMsg().Payload = nil
+	assert.NotPanics(t, func() {
+		_ = sMsg.String()
+	})
 
 	sMsg = &SignedGossipMessage{
 		GossipMessage: &GossipMessage{