iothread: check iothread->ctx before aio_context_unref to avoid assertion

if iothread->ctx is set to NULL, aio_context_unref triggers the assertion:
g_source_unref: assertion 'source != NULL' failed.
The patch fixes it.
Signed-off-by: NLin Ma <lma@suse.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20160926052958.10716-1-lma@suse.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

iothread.c

@@ -75,6 +75,9 @@ static void iothread_instance_finalize(Object *obj)
     iothread_stop(obj, NULL);
     qemu_cond_destroy(&iothread->init_done_cond);
     qemu_mutex_destroy(&iothread->init_done_lock);
+    if (!iothread->ctx) {
+        return;
+    }
     aio_context_unref(iothread->ctx);
 }