wifi: iwlwifi: mvm: add a NULL pointer check

We've observed that in some botched firmware restart scenarios
when the firmware crashes again while we're reconfiguring, we
can hit NULL pointer crashes here. The underlying issue is the
botched restart which we need to fix separately, but until we
can do that, don't crash hard here.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230614123446.e47b0192c78f.I67fa9f07cd1c8b3bdc8db25f5e31c1c680c49745@changeid


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