do not calculate tx fees for miner blocks

src/page.h

@@ -1952,12 +1952,20 @@ namespace xmreg {
             // get mixin number
             txd.mixin_no    = get_mixin_no(tx);
 
+            txd.fee = 0;
+            
             if (!coinbase &&  tx.vin.size() > 0)
+            {
+                // check if not miner tx
+                // i.e., for blocks without any user transactions
+                if (tx.vin.at(0).type() != typeid(txin_gen))
                 {
                     // get tx fee
                     txd.fee = get_tx_fee(tx);
                 }
 
+            }
+
             get_payment_id(tx, txd.payment_id, txd.payment_id8);
 
             // get tx size in bytes