Check if the callback is null again after flush()

The call to flush() may result in the _oldestCallback becoming a nullptr.
For instance, the SslConnected class might report an ssl error.
This causes the _oldestCallback to become null.

include/amqpcpp/channelimpl.h

@@ -674,14 +674,17 @@ public:
 
         // flush the queue, which will send the next operation if the current operation was synchronous
         flush();
-        
-        // we are going to call callbacks that could destruct the channel
-        Monitor monitor(this);
 
         // copy the callback (so that it will not be destructed during
         // the "reportSuccess" call, if the channel is destructed during the call)
         auto cb = _oldestCallback;
 
+        // the call to flush might have caused the callback to have been invoked; check once more
+        if (!cb) return false;
+
+        // we are going to call callbacks that could destruct the channel
+        Monitor monitor(this);
+
         // call the callback
         auto next = cb->reportSuccess(std::forward<Arguments>(parameters)...);