fix: guard decryptors map with RWMutex to prevent data race

[sealbro]

Fix data race on session decryptors map

Problem

The session.decryptors map (golibdave/golibdave.go) was accessed concurrently
without synchronization:

• The audio path (Decrypt, MaxDecryptedFrameSize) reads from the map on
  per-frame decode goroutines.
• The gateway-event path (AddUser, RemoveUser) mutates the map when users
  join or leave the voice channel.

A read on the audio goroutine racing with a write on the event goroutine is a
data race on a Go map, which can panic ("concurrent map read and map write") or
corrupt the map.

Fix

Guard the decryptors map with a sync.RWMutex

[topi314]

I assume you ran into this

[topi314]

I would generally recommend to use https://github.com/thomas-vilte/dave-go
since it doesnt require cgo

[sealbro]

Yep, faced it in integration tests with -race flag.
thanks will check dave-go 👍