cfg80211: fix NULL pointer deference in reg_device_remove()

We won't ever get here as regulatory_hint_core() can only fail
on -ENOMEM and in that case we don't initialize cfg80211 but this is
technically correct code.

This is actually good for stable, where we don't check for -ENOMEM
failure on __regulatory_hint()'s failure.

Cc: stable@kernel.org
Reported-by: Quentin Armitage <Quentin@armitage.org.uk>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/reg.c

@@ -2095,10 +2095,11 @@ int set_regdom(const struct ieee80211_regdomain *rd)
 /* Caller must hold cfg80211_mutex */
 void reg_device_remove(struct wiphy *wiphy)
 {
-	struct wiphy *request_wiphy;
+	struct wiphy *request_wiphy = NULL;
 
 	assert_cfg80211_lock();
 
+	if (last_request)
 		request_wiphy = wiphy_idx_to_wiphy(last_request->wiphy_idx);
 
 	kfree(wiphy->regd);