demux_cue: fix incorrect dirname

[MSViswanth]

When opening cue files which have audio files in a subdirectory, instead of using the path from the cue file, mpv uses the path where the cue file is openend from. So, it always defaults to ./<filename>.<ext>.

Example

Consider the following Test.cue.

PERFORMER "Blue Deer"
TITLE "Tiny Shell"
FILE "Disc 1/Tiny Shell - Blue Deer, Nyles Lannon.mp3" MP3
  TRACK 01 AUDIO
    TITLE "Tiny Shell"
    PERFORMER "Blue Deer, Nyles Lannon"
    INDEX 01 00:00:00

My directory structure

.
├── Disc 1
│   └── Tiny Shell - Blue Deer, Nyles Lannon.mp3
└── Test.cue

mpv crashes like this.

[file] Cannot open file './Tiny Shell - Blue Deer, Nyles Lannon.mp3': No such file or directory
Failed to open ./Tiny Shell - Blue Deer, Nyles Lannon.mp3.
Could not open source './Tiny Shell - Blue Deer, Nyles Lannon.mp3'!
CUE: No useful audio filename in .cue file found, trying with 'Test.cue' instead!
CUE: Could not open audio file!
File tags:
 Performer: Blue Deer
 Title: Tiny Shell
No video or audio streams selected.
Exiting... (Errors when loading file)

Audio file is from YouTube's Audio Library.

[llyyr]

This isn't the right fix, this will break dirname extraction in the opposite direction. That is, cue file is in the same level as the audio file and the CWD isn't the same directory too

[llyyr]

Please try #17868

[MSViswanth]

Please try #17868

This works. Your change looks a lot cleaner.