monitor: guard iothread access by mon->use_io_thread

monitor_resume() and monitor_suspend() both want to
"kick" the I/O thread if it is there, but in
monitor_suspend() lacked the use_io_thread flag condition.
This is required when we later only spawn the thread on
first use.

Signed-off-by: Wolfgang Bumiller <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>

monitor.c

@@ -4292,7 +4292,7 @@ int monitor_suspend(Monitor *mon)
 
     atomic_inc(&mon->suspend_cnt);
 
-    if (monitor_is_qmp(mon)) {
+    if (monitor_is_qmp(mon) && mon->use_io_thread) {
         /*
          * Kick I/O thread to make sure this takes effect.  It'll be
          * evaluated again in prepare() of the watch object.