Skip incomplete Sitemap in robots.txt and warn

Sitemap values should be fully qualified URLs; LinkChecker may not
resolve relative paths correctly.

linkcheck/cache/robots_txt.py

@@ -16,10 +16,13 @@
 """
 Cache robots.txt contents.
 """
+import urllib.parse
+
 from .. import robotparser2
 from ..containers import LFUCache
 from ..decorators import synchronized
 from ..lock import get_lock
+from .. import log, LOG_CACHE
 
 
 # lock objects
@@ -70,6 +73,10 @@ class RobotsTxt:
         if not rp.sitemap_urls or not url_data.allows_simple_recursion():
             return
         for sitemap_url, line in rp.sitemap_urls:
+            if not urllib.parse.urlparse(sitemap_url).scheme:
+                log.warn(LOG_CACHE, _("Relative Sitemap %s in %s discarded"),
+                         sitemap_url, roboturl)
+                continue
             url_data.add_url(sitemap_url, line=line)
 
     @synchronized(robot_lock)