fix: add bounds check for /proc/[pid]/stat fields in fillFromTIDStat

[Yanhu007]

Fixes #2070

Problem

fillFromTIDStatWithContext accesses fields[4], fields[7], fields[14], fields[15], fields[18], and fields[22] without bounds checking. This causes an index-out-of-range panic when /proc/[pid]/stat has fewer than 23 fields.

The function already guards fields[42] at line 1073:

if len(fields) > 42 {
    iotime, err = strconv.ParseFloat(fields[42], 64)
} else {
    iotime = 0 // e.g. SmartOS containers
}

But the same protection is missing for the lower-indexed fields.

Fix

Add an early bounds check:

if len(fields) < 23 {
    return 0, 0, nil, 0, 0, 0, nil, fmt.Errorf("malformed stat file: expected at least 23 fields, got %d", len(fields))
}

This complements the fix in #1995 which added a similar check in a different code path.

[shirou]

LGTM. Minimal and correct fix — the < 23 threshold matches the highest low-index access (fields[22]), and the early return prevents the panic cleanly.

Thanks for the fix!