Refuse to retransmit transactions without vins

Versions of bitcoin before 0.8.6 have a bug that inserted
empty transactions into the vtxPrev in the wallet, which will cause the node to be
banned when retransmitted, hence add a check for !tx.vin.empty()
before RelayTransaction.

src/wallet.cpp

@@ -847,7 +847,10 @@ void CWalletTx::RelayWalletTransaction()
 {
     BOOST_FOREACH(const CMerkleTx& tx, vtxPrev)
     {
-        if (!tx.IsCoinBase())
+        // Important: versions of bitcoin before 0.8.6 had a bug that inserted
+        // empty transactions into the vtxPrev, which will cause the node to be
+        // banned when retransmitted, hence the check for !tx.vin.empty()
+        if (!tx.IsCoinBase() && !tx.vin.empty())
             if (tx.GetDepthInMainChain() == 0)
                 RelayTransaction((CTransaction)tx, tx.GetHash());
     }