Revert "If there is no destination (i.e. CommandType.Done), just skip it."

This reverts commit 7f9fb67.

Author: tmatthey

src/NetMQ/Core/IOThread.cs

@@ -91,7 +91,8 @@ public void Ready()
             // Process all available commands.
             while (m_mailbox.TryRecv(out Command command))
             {
-                command.Destination?.ProcessCommand(command);
+                Assumes.NotNull(command.Destination);
+                command.Destination.ProcessCommand(command);
             }
         }
 

src/NetMQ/Core/Reaper.cs

@@ -126,8 +126,10 @@ public void InEvent()
                 if (!m_mailbox.TryRecv(0, out Command command))
                     break;
 
+                Assumes.NotNull(command.Destination);
+
                 // Process the command.
-                command.Destination?.ProcessCommand(command);
+                command.Destination.ProcessCommand(command);
             }
         }
 

src/NetMQ/Core/SocketBase.cs

@@ -1261,7 +1261,8 @@ private void ProcessCommands(int timeout, bool throttle, CancellationToken cance
             // Process all the commands available at the moment.
             while (found)
             {
-                command.Destination?.ProcessCommand(command);
+                Assumes.NotNull(command.Destination);
+                command.Destination.ProcessCommand(command);
                 found = m_mailbox.TryRecv(0, out command);
             }