iwlwifi: mvm: Fix possible NULL dereference

In __iwl_mvm_remove_time_event() check that 'te_data->vif' is NULL
before dereferencing it.

Fixes: 7b3954a1 ("iwlwifi: mvm: Explicitly stop session protection before unbinding")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ilan Peer <ilan.peer@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.20210907143156.e80e52167d93.Ie2247f43f8acb2cee6dff5b07a3947c79a772835@changeid