Exempt unspendable transaction outputs from dust checks

Since unspendable outputs can't be spent, there is no threshold at which it would be uneconomic to spend them.

This primarily targets transaction outputs with `OP_RETURN`.

---

Initially based on:

commit 9cf0ae26350033d43d5dd3c95054c0d1b1641eda
Author: zathras-crypto <zathrasc@gmail.com>
Date:   Wed Mar 25 02:04:02 2015 -0700

Changes:

- cherry-picked on top of bitcoin:master
- added RPC test for fundrawtransaction

qa/rpc-tests/fundrawtransaction.py

@@ -524,6 +524,22 @@ class RawTransactionsTest(BitcoinTestFramework):
         self.sync_all()
         assert_equal(oldBalance+Decimal('50.19000000'), self.nodes[0].getbalance()) #0.19+block reward
 
+        #####################################################
+        # test fundrawtransaction with OP_RETURN and no vin #
+        #####################################################
+
+        rawtx   = "0100000000010000000000000000066a047465737400000000"
+        dec_tx  = self.nodes[2].decoderawtransaction(rawtx)
+
+        assert_equal(len(dec_tx['vin']), 0)
+        assert_equal(len(dec_tx['vout']), 1)
+
+        rawtxfund = self.nodes[2].fundrawtransaction(rawtx)
+        dec_tx  = self.nodes[2].decoderawtransaction(rawtxfund['hex'])
+
+        assert_greater_than(len(dec_tx['vin']), 0) # at least one vin
+        assert_equal(len(dec_tx['vout']), 2) # one change output added
+
 
 if __name__ == '__main__':
     RawTransactionsTest().main()

src/primitives/transaction.h

@@ -272,10 +272,13 @@ public:
         // which has units satoshis-per-kilobyte.
         // If you'd pay more than 1/3 in fees
         // to spend something, then we consider it dust.
-        // A typical txout is 34 bytes big, and will
+        // A typical spendable txout is 34 bytes big, and will
         // need a CTxIn of at least 148 bytes to spend:
-        // so dust is a txout less than 54 satoshis
+        // so dust is a spendable txout less than 54 satoshis
         // with default minRelayTxFee.
+        if (scriptPubKey.IsUnspendable())
+            return 0;
+
         size_t nSize = GetSerializeSize(SER_DISK,0)+148u;
         return 3*minRelayTxFee.GetFee(nSize);
     }