misc: liquid_dock: Check for NULL notifier parameter

Check nb parameter of register_liquid_dock_notify() in
order to avoid a potential NULL pointer reference if passed
to blocking_notifier_chain_register().

Change-Id: I8d2971d32a1562559e7b27b021efd688beec5494
Signed-off-by: Jack Pham <jackp@codeaurora.org>