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/sendcoinsentry.cpp

@@ -101,20 +101,6 @@ bool SendCoinsEntry::validate()
     // Check input validity
     bool retval = true;
 
-    if(!ui->payAmount->validate())
-    {
-        retval = false;
-    }
-    else
-    {
-        if(ui->payAmount->value() <= 0)
-        {
-            // Cannot send 0 coins or less
-            ui->payAmount->setValid(false);
-            retval = false;
-        }
-    }
-
     if(!ui->payTo->hasAcceptableInput() ||
        (model && !model->validateAddress(ui->payTo->text())))
     {
@@ -122,6 +108,17 @@ bool SendCoinsEntry::validate()
         retval = false;
     }
 
+    if(!ui->payAmount->validate())
+    {
+        retval = false;
+    }
+
+    // Reject dust outputs:
+    if (retval && GUIUtil::isDust(ui->payTo->text(), ui->payAmount->value())) {
+        ui->payAmount->setValid(false);
+        retval = false;
+    }
+
     return retval;
 }