(url-cookie-host-can-set-p): Protect against zero-length domains

(url-cookie-host-can-set-p): Protect against zero-length domains

* lisp/url/url-cookie.el (url-cookie-host-can-set-p): Protect
against zero-length domains.

Backtrace of a real-world site that triggers a bug:

Debugger entered--Lisp error: (args-out-of-range "" 0)
  url-cookie-host-can-set-p("www.washingtonpost.com" "")
  url-cookie-handle-set-cookie("utm_term=0;Expires=Thursday,
  01-January-1970 00:00:00 GMT; path=/; domain=")
  url-http-handle-cookies()