Crash with undo

[Dekel Tsur]

Open the attached file. Press delete 3 times, and then press undo twice, 
and LyX will crash.
Also, after one undo, the display on screen is incorrect (only the c is shown,
while the DVI output both the 'b' and 'c' are shown).
The same crash also happen in 1.2.2cvs