Endpoint used for downloading fails for video with ID awjBLm41xJI when using bestvideo+bestaudio.

[DavidBerdik]

I have found a scenario in which trying to use the endpoint for downloading a video using bestvideo+bestaudio fails.

To reproduce:

1. Run youtube-dl-web.
2. Browse directly to the following URL: http://localhost/api/dl/awjBLm41xJI?f=bestvideo%2Bbestaudio
3. Instead of the download process beginning, you will receive the following error text: Internal Server Error
4. Check the log of the youtube-dl-web-server container. The log will contain the following stack trace which was generated from the failed download attempt.

[awjBLm41xJI]: requested with format bestvideo+bestaudio and subs None, configuring...
[awjBLm41xJI]: sending stream
[awjBLm41xJI]: stream will be merged (bestvideo+bestaudio)
[awjBLm41xJI]: end of data (no error reported)
cleanup[awjBLm41xJI]: killing downloader process (PID: 62)
INFO:     172.19.0.2:55686 - "GET /dl/awjBLm41xJI?f=bestvideo%2Bbestaudio HTTP/1.0" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/uvicorn/protocols/http/h11_impl.py", line 377, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/usr/local/lib/python3.9/site-packages/uvicorn/middleware/proxy_headers.py", line 75, in __call__
    return await self.app(scope, receive, send)
  File "/usr/local/lib/python3.9/site-packages/fastapi/applications.py", line 212, in __call__
    await super().__call__(scope, receive, send)
  File "/usr/local/lib/python3.9/site-packages/starlette/applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/usr/local/lib/python3.9/site-packages/starlette/middleware/errors.py", line 181, in __call__
    raise exc
  File "/usr/local/lib/python3.9/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "/usr/local/lib/python3.9/site-packages/starlette/exceptions.py", line 82, in __call__
    raise exc
  File "/usr/local/lib/python3.9/site-packages/starlette/exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "/usr/local/lib/python3.9/site-packages/starlette/routing.py", line 656, in __call__
    await route.handle(scope, receive, send)
  File "/usr/local/lib/python3.9/site-packages/starlette/routing.py", line 259, in handle
    await self.app(scope, receive, send)
  File "/usr/local/lib/python3.9/site-packages/starlette/routing.py", line 61, in app
    response = await func(request)
  File "/usr/local/lib/python3.9/site-packages/fastapi/routing.py", line 226, in app
    raw_response = await run_endpoint_function(
  File "/usr/local/lib/python3.9/site-packages/fastapi/routing.py", line 159, in run_endpoint_function
    return await dependant.call(**values)
  File "/home/user/app/./server.py", line 23, in api_dl
    first_chunk = await stream.__anext__() # peek first chunk
StopAsyncIteration

I have found that this seems to happen for some audio and video combinations when using the web UI as well. For example, here is a screenshot showing the configuration I attempted to use for this video through the web UI.