Avoid asserts when painting empty wxGridColLabelWindow

When the grid is empty, XToCol() has no choice but to return invalid
index (because there are no valid ones) when it's called from the
drawing code and passing such indices to GetColPos() results in an
assert, which is fatal inside wxEVT_PAINT handler.

Just short-circuit all this code by not doing anything in the case of
empty grid. This is sloppy, but simpler and more robust than any
alternatives.

Closes #18629.

src/generic/grid.cpp

@@ -1813,6 +1813,11 @@ void wxGridColLabelWindow::OnPaint( wxPaintEvent& WXUNUSED(event) )
     //
     // m_owner->PrepareDC( dc );
 
+    // Column indices become invalid when the grid is empty, so avoid doing
+    // anything at all in this case.
+    if ( m_owner->GetNumberCols() == 0 )
+        return;
+
     int x, y;
     wxGridWindow *gridWindow = IsFrozen() ? m_owner->m_frozenColGridWin :
                                             m_owner->m_gridWin;