vnc: fix coverity warning

vnc_display_local_addr will not be called with an invalid display id.
Add assert() to silence coverity warning about a null pointer dereference.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

@@ -3248,6 +3248,7 @@ char *vnc_display_local_addr(const char *id)
 {
     VncDisplay *vs = vnc_display_find(id);
 
+    assert(vs);
     return vnc_socket_local_addr("%s:%s", vs->lsock);
 }