Fix Delete() for invalid PID values

[priyamkarn]

Summary

Fixes #778

This PR fixes Delete() incorrectly treating containers with invalid PIDs (Pid <= 0) as running.

Changes

• Return false from isRunning() when Pid <= 0.
• Prevent syscall.Kill() from being invoked with invalid PID values.

Why

InitialSetup() persists container state with Pid = -1 before container creation completes. If creation fails before the PID is updated, Delete() eventually calls isRunning(), which previously executed syscall.Kill(-1, 0). On Unix systems, this can incorrectly succeed and cause the container to be considered running, preventing cleanup.

With this change, containers in an incomplete state (Pid <= 0) are correctly treated as not running, allowing Delete() to clean them up.

[netlify]

✅ Deploy Preview for urunc canceled.

Name	Link
🔨 Latest commit	d86c8f9
🔍 Latest deploy log	https://app.netlify.com/projects/urunc/deploys/6a3d57b07842bb000839df2e

[priyamkarn]

The guard short-circuits before ever calling syscall.Kill when Pid <= 0. This covers both the reported case (Pid == -1, written by InitialSetup) and the degenerate Pid == 0 case. The Hedge branch is intentionally untouched — it never consults the PID at all, so it's not affected.