FindAdv: Avoid crash finding char at end of inset

Testcase without this patch:
1.) open de/Additional.lyx
2.) goto 6.1 Astronomy & Astrophysics
3.) open the index
4.) find advaced
	a.) not ignoring format
	b.) regex = .+
	c.) language of regex: English
	4.) search next
The seach finds the next break (which is outside of the index)
The following try to display the selection leads to crash