ACPI: video: Fix NULL pointer dereference in acpi_video_bus_add()

acpi_video_bus_add_notify_handler() could free video->input and
set it to NULL on failure, but this failure would be missed in its
caller acpi_video_bus_add(). As a result, when an error happens in
acpi_dev_install_notify_handler(), acpi_video_bus_add() would call
acpi_video_bus_remove_notify_handler(), where a potential NULL pointer
video->input is dereferenced in input_unregister_device().

Fix this by adding a return value check and adjusting the following
error handling code.

Fixes: 6f701681

 ("ACPI: video: Install Notify() handler directly")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>