wifi: mac80211: add consistency check for compat chandef

Add NULL check for compat variable to avoid crash in
cfg80211_chandef_compatible() if it got called with
some mixed up channel context where not all the users
compatible with each other, which shouldn't happen.

Signed-off-by: Anjaneyulu <pagadala.yesu.anjaneyulu@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230616094948.ae0f10dfd36b.Iea98c74aeb87bf6ef49f6d0c8687bba0dbea2abd@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>