MXS-2464: Fix crash on failed session command

If the master fails when a session command is being executed with
delayed_retry enabled, a null query would get placed into the query
queue. This change simply prevents the crash and closes the session even
though the query could be retried.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -945,7 +945,7 @@ void RWSplitSession::handleError(GWBUF* errmsgbuf,
                     m_expected_responses--;
                     errmsg += " Lost connection to master server while waiting for a result.";
 
-                    if (can_retry_query())
+                    if (m_current_query.get() && can_retry_query())
                     {
                         can_continue = true;
                         retry_query(m_current_query.release());