bug#56682: locked narrowing

[Juri Linkov]

> 2. after starting Isearch at a large column number,
>    Emacs hangs up indefinitely, e.g. with
>    'M-g TAB 10000000 RET C-s' then even C-g doesn't get out.
>    Debugging shows that the problem is in 'isearch-update'
>    where the call to 'pos-visible-in-window-group-p' doesn't return.
>    When this call is removed, the search is instantaneous.

There is another side-effect: after evaluating manually
'M-: (pos-visible-in-window-p) RET' at the position 1000000,
point moves to the position 749873 afterwards.  It seems
this is related to locked narrowing, because point doesn't jump
after customizing long-line-locked-narrowing-region-size to 0.