Default filename won't work when saving videos from long tweets

Make sure you are using the latest version: run `youtube-dl --version` and ensure your version is 2018.10.05. If it's not, read this FAQ entry and update. Issues with outdated version will be rejected.

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What is the purpose of your issue?

[x] Bug report (encountered problems with youtube-dl)
[ ] Site support request (request for adding support for a new site)
[ ] Feature request (request for a new functionality)
[ ] Question
[ ] Other

The following sections concretize particular purposed issues, you can erase any section (the contents between triple ---) not applicable to your issue

If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output as follows:

Add the -v flag to your command line you run youtube-dl with (youtube-dl -v <your command line>), copy the whole output and insert it here. It should look similar to one below (replace it with your log inserted between triple ```):

PS C:\Other Programs\youtube-dl> .\youtube-dl.exe -v https://twitter.com/RavenTest/status/1053354601607561
217
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://twitter.com/RavenTest/status/1053354601607561217']
[debug] Encodings: locale cp1252, fs mbcs, out cp850, pref cp1252
[debug] youtube-dl version 2018.10.05
[debug] Python version 3.4.4 (CPython) - Windows-10-10.0.17134
[debug] exe versions: ffmpeg 4.0.2, ffprobe 4.0.2
[debug] Proxy map: {}
[twitter] 1053354601607561217: Downloading webpage
[twitter:card] 1053354601607561217: Downloading webpage
[twitter:card] 1053354601607561217: Downloading guest token
[twitter:card] 1053354601607561217: Downloading JSON metadata
[twitter:card] 1053354601607561217: Downloading m3u8 information
[debug] Default format spec: bestvideo+bestaudio/best
[debug] Invoking downloader on 'https://video.twimg.com/ext_tw_video/1053354339530653697/pu/pl/1280x720/6nenL5yEtmNhiMYj.m3u8'
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 7
[download] Destination: RavenTest - Here is a tweet with a video attached but also lots and lots of text in its body, enough text that Windows filesystems will produce the bug where youtube-dl isn't able to write .part fi...-1053354601607561217.mp4
ERROR: unable to open for writing: [Errno 2] No such file or directory: "RavenTest - Here is a tweet with a video attached but also lots and lots of text in its body, enough text that Windows filesystems will produce the bug where youtube-dl isn't able to write .part fi...-1053354601607561217.mp4.part-Frag0.part"
Traceback (most recent call last):
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmp5xdqinp2\build\youtube_dl\downloader\http.py", line 250, in download
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmp5xdqinp2\build\youtube_dl\utils.py", line 457, in sanitize_open
FileNotFoundError: [Errno 2] No such file or directory: "RavenTest - Here is a tweet with a video attached but also lots and lots of text in its body, enough text that Windows filesystems will produce the bug where youtube-dl isn't able to write .part fi...-1053354601607561217.mp4.part-Frag0.part"

Description of your issue, suggested solution and other information

When using youtube-dl to save a video attached to a tweet, the body of the tweet is used as the filename -- however, this can produce invalid filenames, at least on Windows. Using -o myfilename.mp4 is enough to work around this, but it would be nice if that weren't necessary :)

Note that this problem doesn't occur with very very small videos—the bug only happens when it tries to create .part files, not the actual video files themselves.

In short: youtube-dl.exe https://twitter.com/RavenTest/status/1053357651101470729 WORKS (short video on long tweet)

youtube-dl.exe https://twitter.com/RavenTest/status/1053359277916721154 WORKS (long video on short tweet)

youtube-dl.exe https://twitter.com/RavenTest/status/1053354601607561217 does NOT work (long video on long tweet with default filename)

youtube-dl.exe -o test.mp4 https://twitter.com/RavenTest/status/1053354601607561217 WORKS (long video on long tweet with manual filename)

ytdl-org / youtube-dl