crash on nil in contributions() when needing query-continue

[abartov]

I was getting a crash with users having above 500 contributions (therefore requiring query-continue), seemingly because of this line: res, ucstart = make_api_request(form_data, '//query-continue/usercontribs/@ucstart')

I think I have fixed this, and am submitting a pull request with my patch.

[jpatokal]

Great, thanks!