quiet mode for run_async method might cause ffmpeg process to stick.

What I experienced

Recently, I was using this module in my project for decoding video into pipe as well as encoding target frames into a result video. But I found that setting the parameter "quiet" of the encoder process to True (which set both stdout and stderr to PIPE) will cause the ffmpeg process to stick at some stage ( and in this case, I cannot write anything into its stdin). If I leave "quiet" to default or set both pipe_stdout and pipe_stderr to False, everything goes well.

What I tried

I tried to edit the run_async function, changing it from

args = compile(stream_spec, cmd, overwrite_output=overwrite_output)
    stdin_stream = subprocess.PIPE if pipe_stdin else None
    stdout_stream = subprocess.PIPE if pipe_stdout or quiet else None
    stderr_stream = subprocess.PIPE if pipe_stderr or quiet else None
    return subprocess.Popen(
        args, stdin=stdin_stream, stdout=stdout_stream, stderr=stderr_stream)

to

 args = compile(stream_spec, cmd, overwrite_output=overwrite_output)
    stdin_stream = subprocess.PIPE if pipe_stdin else None
    stdout_stream = subprocess.PIPE if pipe_stdout else None
    stderr_stream = subprocess.PIPE if pipe_stderr else None
    if quiet:
        stdout_stream = stderr_stream = subprocess.DEVNULL
    return subprocess.Popen(
        args, stdin=stdin_stream, stdout=stdout_stream, stderr=stderr_stream)

Then, using this modified version of ffmpeg-python in my project, it works without any problems.

After all, I don't think this issue is solved, for it will stick again if setting either pipe_stdout or pipe_stderr (I confirmed).

My forked version works. Should I make a pull request? Or do it only when we find the real reason for this issue.