Correct wxGrid column labels when using SetTable() with native header.

Ensure that the table pointer inside wxGrid is updated before initializing the
native column header as it uses wxGrid::GetColLabelValue() which doesn't work
correctly without a valid table pointer.

Closes #16399.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76975 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775