The inserted Info frame reused the first audio frame's header, and so its size.
A VBR encode at 22050Hz that starts on silence opens with an 8kbps frame of 26
bytes, while the tag needs 33, so building the frame panicked with an index out
of range. Reproduced with ffmpeg -ar 22050 -q:a 9 on a quiet input.
The frame now keeps the first frame's version, sample rate and channel mode but
raises the bitrate until the frame is big enough for the tag, which is what
ffmpeg's own mp3 muxer does. The frame count is unaffected, and decoded audio of
the reproducing file is byte-identical after the patch.
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