Do not save an invalid network block in wpa_supplicant.conf
wpa_supplicant is going to reject a configuration file that uses
WPA/WPA2-Personal (the default key_mgmt), but does not define
passphrase/PSK. Refuse to save such a configuration to avoid getting
stuck with a configuration that wpa_supplicant will reject.
Signed-hostap: Jouni Malinen <j@w1.fi>