fix(convert): prevent argv out-of-bounds write and use PID-specific workspace

[aki1770-del]

Problem

dlt-convert -t <tarball> has two security issues:

1. argv out-of-bounds write (CWE-131 / stack corruption)

After extracting files to the workspace directory, the code overwrites argc:

n = scandir(DLT_CONVERT_WS, &files, NULL, alphasort);
argc = optind + (n - 2);   // argc extended based on files in /tmp

A subsequent loop then writes to argv[index] using the inflated argc:

for (index = optind; index < argc; index++) {
    ...
    argv[index] = tmp_filename;   // index may exceed original argc!
}

If a malicious local user pre-populates /tmp/dlt_convert_workspace/ with extra files before execution, n is inflated, argc is set beyond the original argv array bounds, and argv[index] writes corrupt the stack — leading to a stack overflow as demonstrated with AddressSanitizer in #793.

2. Predictable workspace path (CWE-377)

The fixed path /tmp/dlt_convert_workspace/ is world-writable and predictable. Any local user can pre-create it with arbitrary contents before dlt-convert is run, triggering the overflow or injecting unexpected .dlt files.

Fixes #793.

Fix

For (1): Introduce a current_file pointer. In the processing loop, set it to tmp_filename in tarball mode or argv[index] otherwise. Remove argv[index] = tmp_filename entirely — the original argv array is never written to.

For (2): Use a PID-specific workspace path /tmp/dlt_convert_ws_<pid>/ (format string DLT_CONVERT_WS_FMT). The path is unique per invocation and not guessable by other local processes.

Testing

• Verified with the PoC from [BUG] stack-overflow in dlt-convert.c #793 (pre-populated /tmp/dlt_convert_ws_*/) — no stack overflow occurs
• Normal -t operation with a valid tar file continues to work correctly