[rule34us] add fallback for 'video-cdn1' videos (#4985)

gallery_dl/extractor/rule34us.py

@@ -38,7 +38,11 @@ class Rule34usExtractor(BooruExtractor):
             "height"  : extr(' x ', 'h'),
             "file_url": extr(' src="', '"'),
         }
-        post["md5"] = post["file_url"].rpartition("/")[2].partition(".")[0]
+
+        url = post["file_url"]
+        if "//video-cdn1." in url:
+            post["_fallback"] = (url.replace("//video-cdn1.", "//video."),)
+        post["md5"] = url.rpartition("/")[2].partition(".")[0]
 
         tags = collections.defaultdict(list)
         for tag_type, tag_name in self._find_tags(page):