Fix assert when adding columns to wxGrid using native header.

Fix assert when adding columns to wxGrid using native header.

Change the order of updating m_colWidths array and the native header control
in wxGird::Redimension() to ensure that we can test whether the column is
shown (which results in accessing m_colWidths element at this column index)
from the wxGridHeaderCtrl::SetColumnCount().

Closes #15225.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74063 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775