Fix frame buffer trimming preferences.

Change-Id: I9771fdb10564a2b896904ede4b5b32b0ed523977
Reviewed-on: http://gerrit.dmdirc.com/2634
Reviewed-by: Shane Mc Cormack <shane@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

src/com/dmdirc/addons/ui_swing/SwingController.java

 import com.dmdirc.ui.core.util.URLHandler;
 import com.dmdirc.updater.Version;
 import com.dmdirc.util.validators.NumericalValidator;
+import com.dmdirc.util.validators.OptionalValidator;
 
 import java.awt.Dialog.ModalityType;
 import java.awt.Font;
 
         advanced.addSetting(new PreferencesSetting(
                 PreferencesType.OPTIONALINTEGER,
-                new NumericalValidator(10, -1), "ui", "frameBufferSize",
-                "Window buffer size", "The maximum number of lines in a window"
-                + " buffer",
-                globalConfig, globalIdentity));
+                new OptionalValidator(new NumericalValidator(10, -1)),
+                "ui", "frameBufferSize",
+                "Window buffer size", "The maximum number of lines in a "
+                + "window buffer", globalConfig, globalIdentity));
         advanced.addSetting(new PreferencesSetting(PreferencesType.BOOLEAN,
                 getDomain(), "mdiBarVisibility", "MDI Bar Visibility",
                 "Controls the visibility of the MDI bar",