Audio PSNR values have strange range

[Jing Lu]

An example:

ffmpeg -f lavfi -i 'sine=frequency=1000:sample_rate=44100:duration=1s' -c:a pcm_u8 ~/sineu8.wav
ffmpeg -f lavfi -i 'sine=frequency=1000:sample_rate=44100:duration=1s' -c:a pcm_s16le ~/sines16le.wav
ffmpeg -i ~/sineu8.wav -i ~/sines16le.wav -filter_complex apsnr -f null /dev/null

Outputs:

[Parsed_apsnr_0 @ 0x5562352d60c0] PSNR ch0: 166.661 dB

This issue is reproducible at v6.1.1 (and many versions before it).

The value of 166.661 dB is rather high (over 100); I'd expect the PSNR to be around ~53 dB.