bug#61514: 30.0.50; sadistically long xml line hangs emacs

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  mah@everybody.org,  61514@debbugs.gnu.org
> Date: Mon, 20 Feb 2023 13:49:49 -0500
> 
> > where I can even reduce the regexp down to "[-._[:alnum:]]*\t*=".
> > Looks like we're missing a case in our backtracking-elimination code.
> 
> The patch below fixes the stack overflow.
> [ And thanks Gregory for the yet simpler test cases.  ]
> 
> I don't think we want that for `emacs-29`, but unless there's some
> objection I'll push this to `master`,

Assuming all the regex-emacs-tests and search-tests pass after this
change, please install on emacs-29, and thanks.