iothread: fix crash with invalid properties

-object iothread,id=foo,? will crash qemu:

qemu-system-x86_64:qemu-thread-posix.c:128: qemu_cond_destroy: Assertion `cond->initialized' failed.

Use thread_id != -1 to check if iothread_complete() finished
successfully and the mutex/cond have been initialized.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180821100716.13803-1-marcandre.lureau@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

iothread.c

@@ -110,6 +110,7 @@ static void iothread_instance_init(Object *obj)
     IOThread *iothread = IOTHREAD(obj);
 
     iothread->poll_max_ns = IOTHREAD_POLL_MAX_NS_DEFAULT;
+    iothread->thread_id = -1;
 }
 
 static void iothread_instance_finalize(Object *obj)
@@ -117,6 +118,11 @@ static void iothread_instance_finalize(Object *obj)
     IOThread *iothread = IOTHREAD(obj);
 
     iothread_stop(iothread);
+
+    if (iothread->thread_id != -1) {
+        qemu_cond_destroy(&iothread->init_done_cond);
+        qemu_mutex_destroy(&iothread->init_done_lock);
+    }
     /*
      * Before glib2 2.33.10, there is a glib2 bug that GSource context
      * pointer may not be cleared even if the context has already been
@@ -135,8 +141,6 @@ static void iothread_instance_finalize(Object *obj)
         g_main_context_unref(iothread->worker_context);
         iothread->worker_context = NULL;
     }
-    qemu_cond_destroy(&iothread->init_done_cond);
-    qemu_mutex_destroy(&iothread->init_done_lock);
 }
 
 static void iothread_complete(UserCreatable *obj, Error **errp)
@@ -147,7 +151,6 @@ static void iothread_complete(UserCreatable *obj, Error **errp)
 
     iothread->stopping = false;
     iothread->running = true;
-    iothread->thread_id = -1;
     iothread->ctx = aio_context_new(&local_error);
     if (!iothread->ctx) {
         error_propagate(errp, local_error);