Fix: Undefined behavior when using UI list or tree-view scrollbar

A scrollbar button would be cast to a number-slider button, and values from
this memory used for scrollbar specific calculations. Looks like an error from
809499a3d0.

In practice the error wouldn't be visible, since the actually used value would
by chance be the intended value, from what I can tell. That's because
`uiButNumberSlider.step_size` and `uiButScrollBar.visual_height` have the same
memory offset within the button memory.

source/blender/editors/interface/interface_handlers.cc

@@ -5888,7 +5888,6 @@ static bool ui_numedit_but_SLI(uiBut *but,
                                const bool snap,
                                const bool shift)
 {
-  uiButNumberSlider *slider_but = reinterpret_cast<uiButNumberSlider *>(but);
   float cursor_x_range, f, tempf, softmin, softmax, softrange;
   int temp, lvalue;
   bool changed = false;
@@ -5917,10 +5916,11 @@ static bool ui_numedit_but_SLI(uiBut *but,
     cursor_x_range = BLI_rctf_size_x(&but->rect);
   }
   else if (but->type == ButType::Scroll) {
+    uiButScrollBar *scroll_but = reinterpret_cast<uiButScrollBar *>(but);
     const float size = (is_horizontal) ? BLI_rctf_size_x(&but->rect) :
                                          -BLI_rctf_size_y(&but->rect);
     cursor_x_range = size * (but->softmax - but->softmin) /
-                     (but->softmax - but->softmin + slider_but->step_size);
+                     (but->softmax - but->softmin + scroll_but->visual_height);
   }
   else {
     const float ofs = (BLI_rctf_size_y(&but->rect) / 2.0f);