Fix values for unsetting speed limits.

src/gui/speedlimitdlg.cpp

@@ -56,7 +56,7 @@ long SpeedLimitDialog::askSpeedLimit(bool *ok, QString title, long default_value
         *ok = true;
         int val = dlg.getSpeedLimit();
         if (val <= 0)
-            return -1;
+            return 0;
         return val * 1024;
     }
     else {
@@ -112,4 +112,4 @@ void SpeedLimitDialog::setupDialog(long max_slider, long val) const
     bandwidthSlider->setMaximum(max_slider);
     bandwidthSlider->setValue(val);
     updateSpinValue(val);
-}
+}