ath9k: fix crash with WEP in ad-hoc mode

Commit eed8e22f0133e8278b1f8079fcb452f1f9692f9d added support for using multicast key lookup to support per-vif/sta keys for AP and ad-hoc. Unfortunately, it also introduced a crash in ad-hoc mode when the sta pointer is NULL, which happens when setting up an interface with WEP keys. This patch fixes it by falling back to the assigned key index.

---

@@ -319,6 +319,10 @@ int ath9k_cmn_key_config(struct ath_comm
 			idx = ath_reserve_key_cache_slot(common, key->alg);
 			break;
 		case NL80211_IFTYPE_ADHOC:
+			if (!sta) {
+				idx = key->keyidx;
+				break;
+			}
 			memcpy(gmac, sta->addr, ETH_ALEN);
 			gmac[0] |= 0x01;
 			mac = gmac;