broadcom_sta: Add patch to fix NULL pointer deref

The patch is from the following Gentoo bug:

https://bugs.gentoo.org/show_bug.cgi?id=523326#c24



Built successfully against Linux 3.18.36, 4.4.16 and 4.7.0.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @phreedom, @vcunat