iwlwifi: mvm: avoid possible NULL pointer dereference

When adding a PASN station, the station is added to the list only
if a TK is configured. Otherwise the station pointer should not be
used.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20201008180656.b8a493c168a7.Ie9a0f9dfd9e9c58c603dd06e45151119467a7804@changeid