Window buffer size os now optional

Fixes issue 3984

Change-Id: Ideba8b9ec4f528b41a671c85713bd96c010d3ea9
Reviewed-on: http://gerrit.dmdirc.com/1100
Reviewed-by: Chris Smith <chris@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>

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

@@ -772,7 +772,7 @@ public final class SwingController extends Plugin implements Serializable,
         options.put("neverShow", "Never show");
         options.put("showWhenMaximised", "Show only when windows maximised");
 
-        advanced.addSetting(new PreferencesSetting(PreferencesType.INTEGER,
+        advanced.addSetting(new PreferencesSetting(PreferencesType.OPTIONALINTEGER,
                 new NumericalValidator(10, -1), "ui", "frameBufferSize",
                 "Window buffer size", "The maximum number of lines in a window"
                 + " buffer"));