Don't use uninitialized variables in wxGrid::AutoSize().

Don't use uninitialized variables in wxGrid::AutoSize().

The code in wxGrid::AutoSizeColOrRow() was using uninitialized col/row
variable if the grid had no rows/columns, resulting in assertion failures when
trying to automatically size the columns of an empty grid.

Do initialize them now and also remove duplicate assignments to the variables
which never change inside the loop.

Closes #12206.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64885 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775