The v0.99 tag in the Git repo triggers this behavior:

http://git.kernel.org/?p=git/git.git;a=tag;h=d6602ec5194c87b0fc87103ca4d67251c76f233a

Ideally, we'd allow the tag to be instantiated even though the tagger
field is missing, but this at the very least prevents libgit2 from
crashing.

To test this bug, a new repository has been added based on the test
branch in testrepo.git. It contains a "e90810b" tag that looks like
this:

    object e90810b8df3e80c413d903f631643c716887138d
    type commit
    tag e90810b

    This is a very simple tag.