Some fixes about group identity and AV

[envsh]

This change is 

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✅ iphydf
✅ envsh
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[iphydf]

It failed to build: https://travis-ci.org/TokTok/go-toxcore-c/builds/394161119#L1075

maybe rebase?

[iphydf]

Reviewed 1 of 2 files at r2, 2 of 3 files at r3, 3 of 3 files at r4.
Reviewable status: complete! 1 of 1 LGTMs obtained

[iphydf]

Reviewed 1 of 2 files at r6, 2 of 3 files at r7, 3 of 3 files at r8.
Reviewable status: complete! 1 of 1 LGTMs obtained

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toxav.go, line 318 at r8 (raw file):

}

func (this *Tox) AddAVGroupChat(cbfn cb_audio_ftype) uint32 {

This change needs an update to group_test.go.