cfg80211: fix disassoc while not associated

When trying to disassociate while not associated,
the kernel would crash rather than refusing the
operation, fix this;
Reported-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/mlme.c

@@ -545,6 +545,12 @@ static int __cfg80211_mlme_disassoc(struct cfg80211_registered_device *rdev,
 
 	ASSERT_WDEV_LOCK(wdev);
 
+	if (wdev->sme_state != CFG80211_SME_CONNECTED)
+		return -ENOTCONN;
+
+	if (WARN_ON(!wdev->current_bss))
+		return -ENOTCONN;
+
 	memset(&req, 0, sizeof(req));
 	req.reason_code = reason;
 	req.ie = ie;