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