vnc: fix memleak of the "vnc-worker-output" name

Fixes repeated memory leaks of 18 bytes when using VNC:

    Direct leak of 831024 byte(s) in 46168 object(s) allocated from:
        ...
        #4 0x7f6d2f919bdd in g_strdup_vprintf glib/gstrfuncs.c:514
        #5 0x56085cdcf660 in buffer_init util/buffer.c:59
        #6 0x56085ca6a7ec in vnc_async_encoding_start ui/vnc-jobs.c:177
        #7 0x56085ca6b815 in vnc_worker_thread_loop ui/vnc-jobs.c:240

Fixes: 543b9580 ("vnc: attach names to buffers")
Cc: Gerd Hoffmann <kraxel@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: NPeter Wu <peter@lekensteyn.nl>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180807221830.3844-1-peter@lekensteyn.nl
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/vnc-jobs.c

@@ -193,6 +193,7 @@ static void vnc_async_encoding_start(VncState *orig, VncState *local)
 
 static void vnc_async_encoding_end(VncState *orig, VncState *local)
 {
+    buffer_free(&local->output);
     orig->tight = local->tight;
     orig->zlib = local->zlib;
     orig->hextile = local->hextile;
@@ -278,7 +279,7 @@ static int vnc_worker_thread_loop(VncJobQueue *queue)
         /* Copy persistent encoding data */
         vnc_async_encoding_end(job->vs, &vs);
 
-	qemu_bh_schedule(job->vs->bh);
+        qemu_bh_schedule(job->vs->bh);
     }  else {
         buffer_reset(&vs.output);
         /* Copy persistent encoding data */