PS-9551 fix: Setting MYSQL_COMMAND_LOCAL_THD_HANDLE may crash the server

https://perconadev.atlassian.net/browse/PS-9551

Fixed problem in 'mysql_command_services_imp::set()'.
When user sets the 'MYSQL_COMMAND_LOCAL_THD_HANDLE' option very early after
server startup (when 'srv_session_server_is_available()' still returns false),
 'service->open(nullptr, nullptr)' may return nullptr and it is unsafe to use it afterwards.

Fixed by checking for nullness and returning earlier.

Author: percona-ysorokin

sql/server_component/mysql_command_services_imp.cc

@@ -660,6 +660,8 @@ DEFINE_BOOL_METHOD(mysql_command_services_imp::set,
             mysql_session = service->open(nullptr, nullptr);
           else
             return true;
+          if (mysql_session == nullptr)
+            return true;
           thd = mysql_session->get_thd();
           mcs_ext->is_thd_associated = false;
           Security_context_handle sc;