Repository: youtube-dl
Issue: [Viki] ffmpeg embed subtitles error 
Labels: geo-restricted

### Make sure you are using the *latest* version: run `youtube-dl --version` and ensure your version is *2018.09.01*. If it's not, read [this FAQ entry](https://github.com/rg3/youtube-dl/blob/master/README.md#how-do-i-update-youtube-dl) and update. Issues with outdated version will be rejected.
- [x] I've **verified** and **I assure** that I'm running youtube-dl **2018.09.01**

### Before submitting an *issue* make sure you have:
- [x] At least skimmed through the [README](https://github.com/rg3/youtube-dl/blob/master/README.md), **most notably** the [FAQ](https://github.com/rg3/youtube-dl#faq) and [BUGS](https://github.com/rg3/youtube-dl#bugs) sections
- [x] [Searched](https://github.com/rg3/youtube-dl/search?type=Issues) the bugtracker for similar issues including closed ones
- [x] Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

### 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*

---

```
~/Downloads/youtube-dl --write-sub --sub-format srt --sub-lang en --embed-subs 'https://www.viki.com/videos/1026409v-my-love-from-the-star-episode-1' -v
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'--write-sub', u'--sub-format', u'srt', u'--sub-lang', u'en', u'--embed-subs', u'https://www.viki.com/videos/1026409v-my-love-from-the-star-episode-1', u'-v']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.09.01
[debug] Python version 2.7.13 (CPython) - Linux-4.9.0-3-amd64-x86_64-with-debian-9.1
[debug] exe versions: ffmpeg 3.2.7-1, ffprobe 3.2.7-1
[debug] Proxy map: {}
[viki] 1026409v: Downloading video JSON
[viki] 1026409v: Downloading video streams JSON
[debug] Default format spec: bestvideo+bestaudio/best
[info] Writing video subtitles to: My Love From the Star - Episode 1-1026409v.en.srt
[debug] Invoking downloader on u'https://v4.viki.io/1026409v/1026409v_high_480p_1708160136.mp4?e=1535939464&h=1b543edad20d7f05d2eee1ff5cd5cfec'
[download] Destination: My Love From the Star - Episode 1-1026409v.mp4
[download] 100% of 348.84MiB in 00:01
[ffmpeg] Embedding subtitles in 'My Love From the Star - Episode 1-1026409v.mp4'
[debug] ffmpeg command line: ffmpeg -y -i 'file:My Love From the Star - Episode 1-1026409v.mp4' -i 'file:My Love From the Star - Episode 1-1026409v.en.srt' -map 0 -c copy -map '-0:s' '-c:s' mov_text -map '1:0' '-metadata:s:s:0' 'language=eng' 'file:My Love From the Star - Episode 1-1026409v.temp.mp4'
ERROR:     Last message repeated 1 times
Traceback (most recent call last):
  File "/home33/megamind/Downloads/youtube-dl/youtube_dl/YoutubeDL.py", line 2047, in post_process
    files_to_delete, info = pp.run(info)
  File "/home33/megamind/Downloads/youtube-dl/youtube_dl/postprocessor/ffmpeg.py", line 393, in run
    self.run_ffmpeg_multiple_files(input_files, temp_filename, opts)
  File "/home33/megamind/Downloads/youtube-dl/youtube_dl/postprocessor/ffmpeg.py", line 204, in run_ffmpeg_multiple_files
    raise FFmpegPostProcessorError(msg)
FFmpegPostProcessorError:     Last message repeated 1 times

...
<end of log>
```
I'm trying to download Video from Viki it works perfectly but when i say it to embed subs it throws me the above error tried multiple files same issue persists (issue with both vtt and srt file types) 
I also use embed subs with Viu but it works great there only having issues with VIU