A copy & paste bug, that resulted setting to NULL something else than
shell->lock_surface when that surface was destroyed.
The symptom: let compositor lock down, unlock it, let it lock down
again, and the unlock dialog is never requested again. This bug was
triggered by the previous fix "shell: fix compositor wakeup while
locked".
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>