Correct test for maximal row index in generic wxDataViewCtrl.

Correct test for maximal row index in generic wxDataViewCtrl.

The comparison in EVT_CHAR handler was incorrect for an empty control without
any rows as it subtracting 1 from 0 resulted in UINT_MAX and not -1 as all the
values were unsigned.

Fix this by checking that the new row is valid instead, this is correct for
both signed and unsigned values.

Closes #13356.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68320 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775