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

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: 7b0a0e3c3a88 ("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>