Centre wxSlider value correctly when wxSL_MIN_MAX_LABELS is not used.

r68230 fixed the width of the value label in this case but not its position:
it was left-aligned instead of being centered as it should be.

See #13291.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68236 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/slider.cpp

@@ -490,7 +490,7 @@ void wxSlider::DoMoveWindow(int x, int y, int width, int height)
         int xLabelValue =
             x + minLabelWidth +
             ((width  - (minLabelWidth + maxLabelWidth)) / 2) -
-            (maxLabelWidth / 2);
+            (longestLabelWidth / 2);
 
         int ySlider = y;