Segfault when calling `index.AddAll`

[pjbgf]

We have experienced index.AddAll causing segfaults. The exact same code depending on the machine it is running on and the repository it is adding files to, may consistently result in a segfault from a background thread:
signal: segmentation fault (core dumped).

In other cases, it may not happen as consistently or not happen at all.
The code extract below is what led to the segfault:

	var parentC []*libgit2.Commit
	head, err := headCommit(repo)
	if err == nil {
		defer head.Free()
		parentC = append(parentC, head)
	}

	index, err := repo.Index()
	if err != nil {
		return "", err
	}
	defer index.Free()

	tracelog.Info("adding files to index")
	c := 1
	err = index.AddAll(nil, libgit2.IndexAddDefault, func(s1, s2 string) error {
		tracelog.Info("adding file", "name", s1, "count", c)
		c += 1
		return nil
	})
	if err != nil {
		return "", err
	}
	tracelog.Info("added files to index")

https://github.com/fluxcd/image-automation-controller/blob/235edabafd1ce34c15635d6c6b5aea18d298a4d3/controllers/imageupdateautomation_controller.go#L627

Versions:

• github.com/libgit2/git2go/v33 v33.0.9
• libgit2 v1.3.1

Relates to fluxcd/image-automation-controller#339