plugins/asf: Refactor to conquer

plugins/asf: Refactor to conquer

  * Move fields inside asf_info: streams, length, type. They are fields
    important both to audio and video files

  * Pass asf_info around to frame parsers: we only decide if it's audio
    or video later, when all important frames are already parsed

  * Try not to reach 80 cols in some places

  * Move code to free streams to its own function

  * Remove useless #if 0 code

  * Check if we parsed any stream. Would be a violation of the format if
    info.streams was NULL in the end, but at least we don't crash