avio_open_dir / avio_close_dir seems to be broken

[rudolfs.bundulis]

Summary of the bug:

when using avio_open_dir / avio_close_dir, avio_close_dir manages to close fd 0, thus messing up further open calls. Found this while making a video filter that was enumerating a directory and used these functions for the enumeration. Further open() calls returned 0 and I was not able to understand why. Removing a call to avio_close_dir() fixed that, and then when debugging found that this is what happens:

1) avio_close_dir() calls ffurl_close()
2) ffurl_close() calls ffurl_closep()
3) ffurl_closep() does

if (h->is_connected && h->prot->url_close)

ret = h->prot->url_close(h);

where for file protocol url_close is set to file_close, which
when invoked will call close on c->fd which has never been set in case of a
directory

For me I'll just fallback to plain POSIX calls for this and seems that ffmpeg code base is not using these calls anywhere, but then at least to save others some amount of gray hair this should either be fixed (not sure what is the best approach) or at least calls marked as unsafe?