Adds check to iterfollow to avoid unwanted exception

This is needed to avoid an AttributeError if self.links was not previously populated. 

To reproduce:

1) Make a request that returns a multi object with a limit higher than the number of currently stored items in Zotero.The response will not include a start or last link in the header and the self.links attribute will not get populated.
2) Try to use iterfollow()

Working example:

GET https://api.zotero.org/users/475425/collections?v=3&limit=1

Will get you a header:

Link	<https://api.zotero.org/users/475425/collections?limit=1&start=1&v=3>; rel="next", <https://api.zotero.org/users/475425/collections?limit=1&start=14&v=3>; rel="last", <https://www.zotero.org/users/475425


Failing example:

GET https://api.zotero.org/users/475425/collections?v=3&limit=100

Will get you a header with:

Link	<https://www.zotero.org/users/475425/collections>; rel="alternate"