bug#34641: rx: (or ...) order unpredictable

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Sun, 24 Feb 2019 19:40:33 +0100
> 
> We could determine whether any string is a prefix of another. If not, 
> regexp-opt should be safe to call. Alternatively, this check could be done in 
> regexp-opt (activated by a flag). That would be my preferred short-term 
> solution.

Your preferred solution is fine with me, FWIW.

> (Speaking of regexp-opt, it has another bug that does not affect rx: it 
> returns the empty string if given an empty list of strings. The correct 
> return value is a regexp that never matches anything. Fix it, document it, or 
> turn it into an error?)

Fix it, I think.