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

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

[ Upstream commit e0015f8c26797afc6fb7e5419da9e3ea245cd387 ]

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: d624b47de5de ("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@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>