[sankaku] update pagination end condition (fixes #265)

Pagination over popular listings (`date:...+order:popular") never
terminates, not even on the site itself, and at some point returns the
same results over and over again.

gallery_dl/extractor/sankaku.py

@@ -225,6 +225,11 @@ class SankakuTagExtractor(SankakuExtractor):
 
             next_qs = text.extract(page, 'next-page-url="/?', '"', pos)[0]
             next_id = text.parse_query(next_qs).get("next")
+
+            # stop if the same "next" parameter occurs twice in a row (#265)
+            if "next" in params and params["next"] == next_id:
+                return
+
             params["next"] = next_id or (text.parse_int(ids[-1]) - 1)
             params["page"] = "2"