Fix panic in select_textobject_around (#9832)

Test Document
-------------
```
a)b
```

Steps to Reproduce
------------------
1. %   # select_all
1. ms( # surround_add
1. mam # select_textobject_around

Debug and Release
-----------------
`thread 'main' panicked at 'Attempt to index past end of RopeSlice:
char index 7, RopeSlice char length 6', ropey-1.6.1/src/slice.rs:796:13`

Description
-----------
An index was selected beyond the end of the slice with chars_at. The fix
adds a guard check to `find_nth_open_pair`, like in the other find_nth*
functions.

0 files changed, 0 insertions, 0 deletions