Corrects button bar with button that has an icon

[Shallowmallow]

If you a had a button in a button bar, it would cause problem

<button-bar>
            <button text="Item 1" icon="haxeui-core/styles/default/haxeui_small.png" iconPosition="top" />
 </button-bar>

You would have

../src/gtk/anybutton.cpp(403): assert ""Assert failure"" failed in DoSetBitmapPosition(): invalid position

That's because the bitmapPosition would be set even the value is "enum(0)".

Now, the line seems quite useless, because it seems the basic buttons page in the component explorer work without.