wifi: nl80211: fix NULL-ptr deref in offchan check

If, e.g. in AP mode, the link was already created by userspace
but not activated yet, it has a chandef but the chandef isn't
valid and has no channel. Check for this and ignore this link.

Fixes: 7b0a0e3c ("wifi: cfg80211: do some rework towards MLO link APIs")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230301115906.71bd4803fbb9.Iee39c0f6c2d3a59a8227674dc55d52e38b1090cf@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -8901,7 +8901,7 @@ static bool cfg80211_off_channel_oper_allowed(struct wireless_dev *wdev,
 		struct cfg80211_chan_def *chandef;
 
 		chandef = wdev_chandef(wdev, link_id);
-		if (!chandef)
+		if (!chandef || !chandef->chan)
 			continue;
 
 		/*