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

src/qt/guiutil.h

@@ -36,6 +36,9 @@ namespace GUIUtil
     bool parseBitcoinURI(const QUrl &uri, SendCoinsRecipient *out);
     bool parseBitcoinURI(QString uri, SendCoinsRecipient *out);
 
+    // Returns true if given address+amount meets "dust" definition
+    bool isDust(const QString& address, qint64 amount);
+
     // HTML escaping for rich text controls
     QString HtmlEscape(const QString& str, bool fMultiLine=false);
     QString HtmlEscape(const std::string& str, bool fMultiLine=false);

src/qt/sendcoinsentry.cpp

@@ -101,24 +101,21 @@ bool SendCoinsEntry::validate()
     // Check input validity
     bool retval = true;
 
-    if(!ui->payAmount->validate())
+    if(!ui->payTo->hasAcceptableInput() ||
+       (model && !model->validateAddress(ui->payTo->text())))
     {
+        ui->payTo->setValid(false);
         retval = false;
     }
-    else
-    {
-        if(ui->payAmount->value() <= 0)
+
+    if(!ui->payAmount->validate())
     {
-            // Cannot send 0 coins or less
-            ui->payAmount->setValid(false);
         retval = false;
     }
-    }
 
-    if(!ui->payTo->hasAcceptableInput() ||
-       (model && !model->validateAddress(ui->payTo->text())))
-    {
-        ui->payTo->setValid(false);
+    // Reject dust outputs:
+    if (retval && GUIUtil::isDust(ui->payTo->text(), ui->payAmount->value())) {
+        ui->payAmount->setValid(false);
         retval = false;
     }