mac80211: fix sta_info_destroy(NULL)

sta_info_destroy(NULL) should be valid, but currently isn't because
the argument is dereferenced before the NULL check. There are no
users that currently pass in NULL, i.e. all check before calling the
function, but I want to change that.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>