Fixed free purchase from cashshop NPC exploit (#3605)

src/map/cashshop.cpp

@@ -564,7 +564,7 @@ bool cashshop_buylist( struct map_session_data* sd, uint32 kafrapoints, int n, u
 		return false;
 	}
 
-	if(pc_paycash( sd, totalcash, kafrapoints, LOG_TYPE_CASH ) < 0){
+	if(pc_paycash( sd, totalcash, kafrapoints, LOG_TYPE_CASH ) <= 0){
 		clif_cashshop_result( sd, 0, CASHSHOP_RESULT_ERROR_SHORTTAGE_CASH );
 		return false;
 	}

src/map/npc.cpp

@@ -1420,7 +1420,9 @@ static enum e_CASHSHOP_ACK npc_cashshop_process_payment(struct npc_data *nd, int
 		case NPCTYPE_CASHSHOP:
 			if (cost[1] < points || cost[0] < (price - points))
 				return ERROR_TYPE_MONEY;
-			pc_paycash(sd, price, points, LOG_TYPE_NPC);
+			if (pc_paycash(sd, price, points, LOG_TYPE_NPC) <= 0) {
+				return ERROR_TYPE_MONEY;
+			}
 			break;
 		case NPCTYPE_ITEMSHOP:
 			{