Do not count 0-fee txes for fee estimation (dashpay#3037)

Author: UdjinM6

src/validation.cpp

@@ -883,8 +883,8 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
 
         // This transaction should only count for fee estimation if the
         // node is not behind, and the transaction is not dependent on any other
-        // transactions in the mempool.
-        bool validForFeeEstimation = IsCurrentForFeeEstimation() && pool.HasNoInputsOf(tx);
+        // transactions in the mempool. Also ignore 0-fee txes.
+        bool validForFeeEstimation = (nFees != 0) && IsCurrentForFeeEstimation() && pool.HasNoInputsOf(tx);
 
         // Store transaction in memory
         pool.addUnchecked(hash, entry, setAncestors, validForFeeEstimation);