vnc: Fix fatal crash with vnc reverse mode

Reverse mode is unusable:

	qemu -vnc localhost:5500,reverse

crashes in vnc_refresh_server_surface because some pointers are NULL.

Fix this by calling vnc_dpy_resize (which initializes these pointers)
before calling vnc_refresh.

Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

ui/vnc.c

diff --git a/ui/vnc.c b/ui/vnc.c
index 626b430..610f884 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -2480,6 +2480,7 @@
     vd->timer_interval = VNC_REFRESH_INTERVAL_BASE;
     if (vd->timer == NULL && !QTAILQ_EMPTY(&vd->clients)) {
         vd->timer = qemu_new_timer(rt_clock, vnc_refresh, vd);
+        vnc_dpy_resize(vd->ds);
         vnc_refresh(vd);
     }
 }