Add a simple check to prevent kernel panics from nullptr dereferences

Fixes #77.

core/rtw_mlme.c

@@ -4275,6 +4275,14 @@ static int rtw_rsn_sync_pmkid(_adapter *adapter, u8 *ie, uint ie_len, int i_ent)
 	if (i_ent >= 0) {
 		RTW_INFO(FUNC_ADPT_FMT" append PMKID:"KEY_FMT"\n"
 			, FUNC_ADPT_ARG(adapter), KEY_ARG(sec->PMKIDList[i_ent].PMKID));
+        if (!info.pmkid_list) {
+            /* prevent nullptr dereference when trying to insert a PMKID into 
+             * a frame that did not previously contain one. In order to be minimally
+             * invasive, we just discard requests like these, which might impact
+             * the ability to connect to certain access points, but will at least
+             * prevent the kernel panics */
+            return 0;
+        }
 
 		info.pmkid_cnt = 1; /* update new pmkid_cnt */
 		_rtw_memcpy(info.pmkid_list, sec->PMKIDList[i_ent].PMKID, 16);