cache_file: fix operation if stream size is unknown

Happens when playing from a pipe.

Note that seeking forward doesn't work. It would be possible to create a
workaround for that by reading and skipping data until the target
position is reached (and writing the skipped data into the cache file),
but I'm not sure about that.

Fixes #928.

CC: @mpv-player/stable
stream: don't sleep for reconnecting network if playback is stopped

Also silences the bogus message if that happens.

CC: @mpv-player/stable