Remove zero-length data panic from messaging.Conn.

This check is no longer necessary as the checksum added
to the messaging.Message will catch any data errors.

messaging/client.go

@@ -674,9 +674,9 @@ func (c *Conn) stream(req *http.Request, closing <-chan struct{}) error {
 			return fmt.Errorf("decode: %s", err)
 		}
 
-		// Panic if we received no data.
+		// Log if we received a message with no data.
 		if len(m.Data) == 0 {
-			panic("messaging conn no data recv")
+			c.Logger.Printf("conn recv message w/ no data: type=%x", m.Type)
 		}
 
 		// TODO: Write broker set updates, do not passthrough to channel.