ACPI / event: remove unneeded NULL pointer check

nla_data() cannot return NULL as it simply adds NLA_HDRLEN to the
passed pointer. The NULL pointer check is therefore superfluous.
Remove it.

Found by smatch:
  drivers/acpi/event.c:130 acpi_bus_generate_netlink_event() warn: can
  'event' even be NULL?

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>