msgr: allow users to mark_down a NULL Connection*

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sam Just <sam.just@inktank.com>

src/msg/Messenger.h

@@ -476,6 +476,7 @@ public:
    * discard its outgoing queue, and if the endpoint tries
    * to reconnect they will discard their queue when we
    * inform them of the session reset.
+   * If the Connection* is NULL, this is a no-op.
    * It does not generate any notifications to the Dispatcher.
    *
    * @param con The Connection to mark down.

src/msg/SimpleMessenger.cc

@@ -585,6 +585,8 @@ void SimpleMessenger::mark_down(const entity_addr_t& addr)
 
 void SimpleMessenger::mark_down(Connection *con)
 {
+  if (con == NULL)
+    return;
   lock.Lock();
   Pipe *p = static_cast<Pipe *>(con->get_pipe());
   if (p) {