[extractor] Fix root-relative URLs in MPD (#1006)

Authored by: DigitalDJ

yt_dlp/extractor/common.py

@@ -2622,8 +2622,10 @@ class InfoExtractor(object):
                             base_url = base_url_e.text + base_url
                             if re.match(r'^https?://', base_url):
                                 break
-                    if mpd_base_url and not re.match(r'^https?://', base_url):
-                        if not mpd_base_url.endswith('/') and not base_url.startswith('/'):
+                    if mpd_base_url and base_url.startswith('/'):
+                        base_url = compat_urlparse.urljoin(mpd_base_url, base_url)
+                    elif mpd_base_url and not re.match(r'^https?://', base_url):
+                        if not mpd_base_url.endswith('/'):
                             mpd_base_url += '/'
                         base_url = mpd_base_url + base_url
                     representation_id = representation_attrib.get('id')