Fix an error in how cksum parses checksum files

[frenchua]

When cksum (or sha256sum, etc...) is called with the -c option, and it attempts to parse cheksum files in the "tagged output format" and it encounters the string "((", it crashes due to an error in how it parses the checksum file. GNU coreutils, by contrast, correctly recognizes this line as a syntax error and ignores the line.

This is due to the following two lines of code in the parse_algo_based function in validate.rs:

let par_idx = rest.iter().position(|&b| b == b'(')?;
let sub_case = if rest[par_idx - 1] == b' ' {

If the checksum file contains two consecutive left parentheses, then par_idx is equal to zero and so rest[par_idx - 1] evaluates to rest[-1] which causes the program to crash.

I have added a check to make sure that par_idx is greater than zero because, In my view, the string '((' appearing in a checksum file should be considered a syntax error and the line ignored, just as it does in the GNU cksum/sha256sum program).

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)

[sylvestre]

could you please add a test to make sure we don't regress in the future?
(and that demonstrate the failure you are talking about)
thanks