ASSERT on entering regexp inset when instant preview on

[Tommaso Cucinotta]

Hi all,

I just dropped a ticket about an assertion triggered as follows (on 
current trunk r39819):

0) Activate instant preview from prefs
1) C-S-f (open the find advanced panel)
2) C-m (start math inset)
3) Menu: Insert->Regular Expression->Anything
4) C-Home
5) Cursor right twice

For details, see #7805.

    T.