wifi: iwlwifi: mvm: avoid NULL pointer dereference

In the Linux kernel, the following vulnerability has been resolved: wifi: iwlwifi: mvm: avoid NULL pointer dereference When iterating over the links of a vif, we need to make sure that the pointer is valid (in other words - that the link exists) before dereferncing it. Use for_each_vif_active_link that also does the check.