Empty output file after conversion if source is 5.1 audio (ac3>opus)

[necros]

Summary of the bug: ac3>opus conversion fails with empty output file if source file have 5.1 audio
How to reproduce:

ffmpeg -i 1.ac3 -map_metadata -1 -c:a libopus -b:a 128k 1.opus}}}

ffmpeg version N-118351-ga0a89efd07-20250125 Copyright (c) 2000-2025 the FFmpeg developers
  built with gcc 14.2.0 (crosstool-NG 1.26.0.120_4d36f27)

[libopus @ 000001fd9e5eaa40] Invalid channel layout 5.1(side) for specified mapping family -1.
[aost#0:0/libopus @ 000001fd9e5ea800] [enc:libopus @ 000001fd9c5d9080] Error while opening encoder - maybe incorrect parameters such as bit_rate, rate, width or height.
[af#0:0 @ 000001fd9c5242c0] Error sending frames to consumers: Invalid argument
[af#0:0 @ 000001fd9c5242c0] Task finished with error code: -22 (Invalid argument)
[af#0:0 @ 000001fd9c5242c0] Terminating thread with return code -22 (Invalid argument)
[aost#0:0/libopus @ 000001fd9e5ea800] [enc:libopus @ 000001fd9c5d9080] Could not open encoder before EOF
[aost#0:0/libopus @ 000001fd9e5ea800] Task finished with error code: -22 (Invalid argument)
[aost#0:0/libopus @ 000001fd9e5ea800] Terminating thread with return code -22 (Invalid argument)
[out#0/opus @ 000001fd9c54bb40] Nothing was written into output file, because at least one of its streams received no packets.

[Yogeshwar Velingker]

Duplicate of #5718.