bug#55205: 28.1.50; completion--replace illegally mutates completion candidates

[Daniel Mendler]

> As an aside, please don't use "illegal" unless you mean something that
> is against the law.  GNU Coding Standards frown on using "illegal" for
> something that is merely incorrect or invalid coding practices.

Good to know.

[[However I'd say that the word "illegal" is used precisely right here.
If we talk about "illegal memory access" we talk about memory being
accessed against the rules or ownership of the memory region. In this
case a string is mutated by completing-read, but ownership of the string
in question is not passed to completing-read.]]