wifi: iwlwifi: mvm: check n_ssids before accessing the ssids

In the Linux kernel, the following vulnerability has been resolved: wifi: iwlwifi: mvm: check n_ssids before accessing the ssids In some versions of cfg80211, the ssids poinet might be a valid one even though n_ssids is 0. Accessing the pointer in this case will cuase an out-of-bound access. Fix this by checking n_ssids first.