panic subtract with overflow

[eerimoq]

Hi!

I sometimes get this when playing using ffplay:

thread 'smol-1' panicked at 'attempt to subtract with overflow', /Users/erik/workspace/irl/river/src/rtmp_server.rs:143:52

ffplay says this:

[flv @ 0x14161f2e0] AMF_DATA_TYPE_STRING parsing failed