TRadioGroup: fix resetting ItemIndex to -1 when calling Items.Assign (regression introduced by me in r42772).
This caused ItemIndex being reset to -1 when editing Items in OI.


git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@43223 4005530d-fff6-0310-9dd1-cebe43e6787f