Fix decoding flac using ffmpeg.dll plugin

[c72578]

Flac files could not be decoded anymore using ffmpeg.dll since ffmpeg version 5.0 (4.4 and earlier were OK).

• Using ffmpeg 4.4 or earlier, the following message appeared in the debug output, when decoding flac files using ffmpeg.dll: Invalid return value 0 for stream protocol
• Newer versions of ffmpeg dlls like 5.0 to 6.1 did not show this output anymore, whereas CUETools got stuck at: "Analyzing input file..."
• The problem occurred in av_read_frame() and the readPacketCallback(), where a value of len = 0 was returned at the end of flac files.
• Only decoding of flac files using ffmpeg.dll was affected. Other file formats like aiff, ape, m4a, tta and wv could be decoded properly.
• Return AVERROR_EOF instead of 0 in readPacketCallback()
• Resolves #300

[c72578]

Here is a build for testing, which allows decoding flac files again using the ffmpeg.dll plugin: CUETools_2.2.4_2023-11-17_git_271a0e7_Fix_decoding_flac_using_ffmpeg.dll.zip SHA256: 33ad0c3c8e33f8a269640338caf6493a70c0e2341591ac1fd63c2d1152f3068b

Download links for the required ffmpeg 6.0 dlls can be found e.g. here: https://github.com/gchudov/cuetools.net/pull/276#issuecomment-1516895823

@ha-korth, could you please have a look.