Reset row and columns count in wxRichTextTable::ClearTable().

No real changes, just update the internal variables in ClearTable() to avoid
inconsistent internal state, even if this doesn't seem to result in any
problems for now.

Closes #15190.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74057 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775