ui.c: Make sure to not set an invalid ui_tab_cur in ui_tab_remove()
ui.c: Make sure to not set an invalid ui_tab_cur in ui_tab_remove()

I have honestly no idea when this situation could occur, but I did
stumble upon a crash that was caused by an incorrect value of
ui_tab_cur: http://p.blicky.net/bo9g7

Unfortunately, I am unable to reproduce the crash, so can't tell if this
is was what caused it or not, but I honestly can't find anything else
that may have been responsible. :-(

srijan/ncdc,srijan/ncdc,Tilka/ncdc,srijan/ncdc,Tilka/ncdc