stat: fix overflow on out-of-range octal escape

[harshasiddartha]

What was wrong

stat --printf '\400' (and any \NNN escape from \400 through \777) aborted instead of printing a byte:

$ ./target/debug/coreutils stat --printf '\400' . >/dev/null
thread 'main' panicked at src/uu/stat/src/stat.rs:950:33:
attempt to multiply with overflow
$ echo $?
101

Any build with overflow-checks enabled (debug builds by default, or release with -C overflow-checks) hits this.

Root cause

handle_escape_sequences accumulated the octal escape in a u8:

let mut value = 0u8;
…
value = value * 8 + digit as u8;

Three octal digits can encode values up to 511, so as soon as the accumulated value passes u8::MAX the multiply overflows.

The fix

Accumulate in a u32 — three octal digits reach at most 511, so the arithmetic can no longer overflow — and keep the low byte when building the Token::Byte. That makes \400 print 0x00 and \777 print 0xFF, matching what GNU stat outputs for an out-of-range octal escape. In-range escapes are unaffected. Only --printf is affected: generate_tokens interprets backslash escapes for --printf and treats them literally for -c/--format, matching GNU.

The fix was written from scratch; no GNU source was consulted or copied, only GNU's observable output for these escapes.

How it was tested

• New regression test test_printf_octal_out_of_range in tests/by-util/test_stat.rs asserts that --printf=\400\777 succeeds and emits the bytes 00 ff. It fails on main (the command exits 101) and passes with this change.
• The full stat test suite passes: cargo test --no-default-features --features stat --test tests test_stat → 34 passed, 0 failed.
• cargo clippy -p uu_stat --no-default-features --all-targets is clean, and cargo fmt reports no changes.

Fixes #14346

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/dd/misc. tests/dd/misc is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/df/over-mount-device. tests/df/over-mount-device is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cat/splice is no longer failing!
Congrats! The gnu test tests/cp/cp-a-selinux is no longer failing!
Congrats! The gnu test tests/cut/cut is no longer failing!
Congrats! The gnu test tests/cut/mb-non-utf8 is no longer failing!
Congrats! The gnu test tests/dd/partial-write is no longer failing!
Congrats! The gnu test tests/expand/mb is no longer failing!
Congrats! The gnu test tests/ls/stat-free-symlinks is no longer failing!
Congrats! The gnu test tests/misc/close-stdout is no longer failing!
Congrats! The gnu test tests/mktemp/write-error is no longer failing!
Congrats! The gnu test tests/mv/dir2dir is no longer failing!
Congrats! The gnu test tests/mv/mv-exchange is no longer failing!
Congrats! The gnu test tests/nl/multibyte is no longer failing!
Congrats! The gnu test tests/od/od-float is no longer failing!
Congrats! The gnu test tests/od/od-j is no longer failing!
Congrats! The gnu test tests/ptx/ptx-overrun is no longer failing!
Congrats! The gnu test tests/sort/sort-merge-fdlimit is no longer failing!
Congrats! The gnu test tests/unexpand/mb is no longer failing!
Note: The gnu test tests/dd/fail-ftruncate-fstat was skipped on 'main' but is now failing.

[cakebaker]

Thanks for your PR!