crash in flat selector when pressing tab

[cycloid]

apologies if this is fixed in the new dev

1. load wad (mine has lots of replacement tex's sprites and flats)
2. go to find...and replace
3. select "sector flat"
4. flat to find: select from the list of current flats
5. flat to replace it with: press tab to show all
6. kaboom

[CodeImp]

Known bug. Also see this thread.