sort: fix infinite loop on --files0-from read error

[vitkyrka]

If the read from the --files0-from file results in a permanent error, we end up in an infinite loop.

Clippy will detect this in the future, see rust-lang/rust-clippy#16763.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cp/link-heap is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/cp/link-heap is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[cakebaker]

Thanks for your PR!