[ndr][ndr:embed] failure to correctly expand URL for thumbnail download

[barsnick]

Checklist

• I'm reporting a broken site support
• I've verified that I'm running youtube-dl version 2020.01.01
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones

Verbose log

[barsnick@paradise ~]$ ~/Downloads/youtube-dl-2020.01.01 --verbose --skip-download --write-thumbnail https://www.ndr.de/fernsehen/sendungen/die_nordstory/Heisse-Tage-im-Schnee-Hochsaison-im-Oberharz,sendung983522.html
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--verbose', '--skip-download', '--write-thumbnail', 'https://www.ndr.de/fernsehen/sendungen/die_nordstory/Heisse-Tage-im-Schnee-Hochsaison-im-Oberharz,sendung983522.html']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2020.01.01
[debug] Python version 3.7.5 (CPython) - Linux-5.4.7-200.fc31.x86_64-x86_64-with-fedora-31-Thirty_One
[debug] exe versions: ffmpeg 4.2.2, ffprobe 4.2.2
[debug] Proxy map: {}
[ndr] Heisse-Tage-im-Schnee-Hochsaison-im-Oberharz: Downloading webpage
[ndr:embed] dienordstory1036: Downloading JSON metadata
[ndr:embed] dienordstory1036: Downloading f4m manifest
WARNING: Unable to download f4m manifest: HTTP Error 404: Not Found
[ndr:embed] dienordstory1036: Downloading m3u8 information
[debug] Default format spec: bestvideo+bestaudio/best
[ndr:embed] dienordstory1036: Downloading thumbnail ...
Traceback (most recent call last):
  File "/usr/lib64/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib64/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/__main__.py", line 19, in <module>
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/__init__.py", line 474, in main
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/__init__.py", line 464, in _real_main
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/YoutubeDL.py", line 2018, in download
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/YoutubeDL.py", line 807, in extract_info
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/YoutubeDL.py", line 900, in process_ie_result
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/YoutubeDL.py", line 862, in process_ie_result
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/YoutubeDL.py", line 1643, in process_video_result
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/YoutubeDL.py", line 1854, in process_info
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/YoutubeDL.py", line 2410, in _write_thumbnails
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/YoutubeDL.py", line 2236, in urlopen
  File "/home/barsnick/Downloads/youtube-dl-2020.01.01/youtube_dl/utils.py", line 2149, in sanitized_Request
  File "/usr/lib64/python3.7/urllib/request.py", line 328, in __init__
    self.full_url = url
  File "/usr/lib64/python3.7/urllib/request.py", line 354, in full_url
    self._parse()
  File "/usr/lib64/python3.7/urllib/request.py", line 383, in _parse
    raise ValueError("unknown url type: %r" % self.full_url)
ValueError: unknown url type: '/nachrichten/niedersachsen/braunschweig_harz_goettingen/wurmberg540_v-contentxl.jpg'

Description

The NDR extractor works fine in terms of the actual video, but trips over grabbing the requested thumbnail, due to failure to correctly expand the URL. If I add the base URL from the calling page to the filename presented in the error message, I can download the image (e.g. with a downloader such as wget).

(For the record: There's also a warning about a failure to access an f4m manifest, but that's a different issue.)
(For the record: If requested, the extractor also fails to discover the subtitles, which are located at https://www.ndr.de/fernsehen/sendungen/die_nordstory/ut125516.xml for the given page.)

Broken URL:
https://www.ndr.de/fernsehen/sendungen/die_nordstory/Heisse-Tage-im-Schnee-Hochsaison-im-Oberharz,sendung983522.html

Image reported as unknown URL:
/nachrichten/niedersachsen/braunschweig_harz_goettingen/wurmberg540_v-contentxl.jpg

Actual working thumbnail URL:
https://www.ndr.de/nachrichten/niedersachsen/braunschweig_harz_goettingen/wurmberg540_v-contentxl.jpg