job may be NULL if queue->exit is true. Check
it before dereference job.

Fixes: f31f9c10 ("vnc: add magic cookie to VncState")
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>