vnc: Fix crash on 'info vnc' after 'change vnc none'

vnc_state->display is set to NULL after "change vnc none" but vnc_state itself
is still valid.

(James Ko)

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6010 c046a42c-6fe2-441c-8c8c-71466251a162

vnc.c

@@ -183,7 +183,7 @@ static VncState *vnc_state; /* needed for info vnc */
 
 void do_info_vnc(void)
 {
-    if (vnc_state == NULL)
+    if (vnc_state == NULL || vnc_state->display == NULL)
 	term_printf("VNC server disabled\n");
     else {
 	term_printf("VNC server active on: ");