Don't try to export when we don't have filters and fix mem leak when we cancel

src/filter/kmfilterdialog.cpp

@@ -784,6 +784,10 @@ void KMFilterDialog::slotExportAsSieveScript()
     KMessageBox::information(this, i18n("We cannot convert all KMail filters to sieve scripts but we can try :)"), i18n("Convert KMail filters to sieve scripts"));
     bool wasCanceled = false;
     const QList<MailFilter *> filters = mFilterList->filtersForSaving(false, wasCanceled);
+    if (!filters.isEmpty()) {
+        return;
+    }
+    if (!wasCanceled) {
         QPointer<FilterSelectionDialog> dlg = new FilterSelectionDialog(this);
         dlg->setFilters(filters);
         if (dlg->exec() == QDialog::Accepted) {
@@ -797,6 +801,9 @@ void KMFilterDialog::slotExportAsSieveScript()
             }
         }
         delete dlg;
+    } else {
+        qDeleteAll(filters);
+    }
 }
 
 void KMFilterDialog::slotHelp()