vnc: fix a memory leak in threaded vnc server

VncJobQueue's buffer is intended to be used for
as the output buffer for all operations in this queue,
but unfortunatly.

vnc_async_encoding_start() is in charge of setting this
buffer as the current output buffer, but
vnc_async_encoding_end() was not writting the changes back
to VncJobQueue, resulting in a big and ugly memleak.
Signed-off-by: NCorentin Chary <corentincj@iksaif.net>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

ui/vnc-jobs-async.c

@@ -186,6 +186,8 @@ static void vnc_async_encoding_end(VncState *orig, VncState *local)
     orig->hextile = local->hextile;
     orig->zrle = local->zrle;
     orig->lossy_rect = local->lossy_rect;
+
+    queue->buffer = local->output;
 }
 
 static int vnc_worker_thread_loop(VncJobQueue *queue)