[ie/generic] Fix KVS thumbnail extraction

Closes #8045
Authored by: bashonly

yt_dlp/extractor/generic.py

@@ -2370,7 +2370,7 @@ def _extract_kvs(self, url, webpage, video_id):
             'id': flashvars['video_id'],
             'display_id': display_id,
             'title': title,
-            'thumbnail': thumbnail,
+            'thumbnail': urljoin(url, thumbnail),
             'formats': formats,
         }