Reject dust amounts during validation

Replaces the validation check for "amount == 0" with an isDust check,
so very small output amounts are caught before the wallet
is unlocked, a transaction is created, etc.

src/qt/guiutil.cpp

@@ -148,6 +148,14 @@ bool parseBitcoinURI(QString uri, SendCoinsRecipient *out)
     return parseBitcoinURI(uriInstance, out);
 }
 
+bool isDust(const QString& address, qint64 amount)
+{
+    CTxDestination dest = CBitcoinAddress(address.toStdString()).Get();
+    CScript script; script.SetDestination(dest);
+    CTxOut txOut(amount, script);
+    return txOut.IsDust(CTransaction::nMinRelayTxFee);
+}
+
 QString HtmlEscape(const QString& str, bool fMultiLine)
 {
 #if QT_VERSION < 0x050000