lib/index: Fix memory leak for email addresses without names.

lib/index: Fix memory leak for email addresses without names.

We carefully noted the fact that we had locally allocated the string
here, but then we neglected to free it. Switch to talloc instead
which makes it easier to get the behavior we want. It's simpler since
we can just call talloc_free unconditionally, without having to track
the state of whether we allocated the storage for name or not.