vnc: fix screen updates

Bug was added by 38ee14f4f3.
vnc_jobs_join call is missing in one code path.

Reported-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

@@ -935,6 +935,9 @@ static int vnc_update_client(VncState *vs, int has_dirty, bool sync)
         }
 
         vnc_job_push(job);
+        if (sync) {
+            vnc_jobs_join(vs);
+        }
         vs->force_update = 0;
         return n;
     }