receive-pack: crash when checking with non-exist HEAD

If HEAD of a repository points to a conflict reference, such as: * There exist a reference named 'refs/heads/jx/feature1', but HEAD points to 'refs/heads/jx', or * There exist a reference named 'refs/heads/feature', but HEAD points to 'refs/heads/feature/bad'. When we push to delete a reference for this repo, such as: git push /path/to/bad-head-repo.git :some/good/reference The git-receive-pack process will crash. This is because if HEAD points to a conflict reference, the function `resolve_refdup("HEAD", ...)` does not return a valid reference name, but a null buffer. Later matching the delete reference against the null buffer will cause git-receive-pack crash. Signed-off-by: Jiang Xin <worldhello.net@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>