Fix endless loop in PSK fetching with PSK-from-RADIUS

Fix endless loop in PSK fetching with PSK-from-RADIUS

Commit 05ab9712b9977192b713f01f07c3b14ca4d1ba78 added support for
fetching WPA PSK from an external RADIUS server and changed
hostapd_wpa_auth_get_psk() to always return the RADIUS supplied PSK (if
set) and ignore the prev_psk parameter for iteration. Fix this by
appending the RADIUS supplied PSK to the list iterated by
hostapd_get_psk and thus returning NULL when prev_psk == sta->psk
(RADIUS).

Signed-hostap: M. Braun <michael-dev@fami-braun.de>