When calling `git_note_next`, we end up calling `git_iterator_advance`
but ignore its error code. The intent is that we do not want to return
an error if it returns `GIT_ITEROVER`, as we want to return that value
on the next invocation of `git_note_next`. We should still check for any
other error codes returned by `git_iterator_advance` to catch unexpected
internal errors.

Fix this by checking the function's return value, ignoring
`GIT_ITEROVER`.