nl80211: check nla_nest_start() return value

Coverity pointed out that we might dereference NULL later
if nla_nest_start() returns a failure. This isn't really
true since we'd bomb out before, but we should check the
return value directly, so do that.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -11094,6 +11094,8 @@ void cfg80211_report_wowlan_wakeup(struct wireless_dev *wdev,
 		struct nlattr *reasons;
 
 		reasons = nla_nest_start(msg, NL80211_ATTR_WOWLAN_TRIGGERS);
+		if (!reasons)
+			goto free_msg;
 
 		if (wakeup->disconnect &&
 		    nla_put_flag(msg, NL80211_WOWLAN_TRIG_DISCONNECT))