cfg80211: fix few minor issues in reg_process_hint()

Fix the following issues in reg_process_hint():

1. Add verification that wiphy is valid before processing
   NL80211_REGDOMAIN_SET_BY_COUNTRY_IE.
2. Free the request in case of invalid initiator.
3. Remove WARN_ON check on reg_request->alpha2 as it is not a
   pointer.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>