qemumonitortestutils: Don't crash on non fully initialized test

The qemumonitorjsontest crashed when one of the initialization steps
done before starting the worker thread failed. This patch fixes this by
trying to pthread_join() the thread only after it was created.

tests/qemumonitortestutils.c

@@ -368,7 +368,8 @@ qemuMonitorTestFree(qemuMonitorTestPtr test)
 
     virObjectUnref(test->vm);
 
-    virThreadJoin(&test->thread);
+    if (test->running)
+        virThreadJoin(&test->thread);
 
     if (timer != -1)
         virEventRemoveTimeout(timer);