Summary

Fixes a nil-pointer panic in (*udpConnImpl).Close() when called before Open() has assigned u.conn.

(*connImpl).HandleVoiceStateUpdate unconditionally calls c.udp.Close() whenever a VOICE_STATE_UPDATE arrives with ChannelID == nil (the bot was kicked, moved, or its session was torn down before voice negotiation completed). If the UDP connection was never opened, u.conn is still nil and the bare return u.conn.Close() panics with a SIGSEGV, taking down the entire gateway listener goroutine.

The fix adds a nil-check so Close() is safe to call on an un-opened UDP conn — matching the defensive if c.audioSender != nil { ... } and if c.audioReceiver != nil { ... } guards already present on the surrounding lines of HandleVoiceStateUpdate.

Stack (reproduced against v0.19.5)

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x18 pc=0x10301d654]

goroutine 85 [running]:
github.com/disgoorg/disgo/voice.(*udpConnImpl).Close(...)
        voice/udp_conn.go:401 +0xa4
github.com/disgoorg/disgo/voice.(*connImpl).HandleVoiceStateUpdate(...)
        voice/conn.go:186 +0x110
github.com/disgoorg/disgo/voice.(*managerImpl).HandleVoiceStateUpdate(...)
        voice/manager.go:72 +0xf4
github.com/disgoorg/disgo/bot/handlers.gatewayHandlerVoiceStateUpdate(...)
        bot/handlers/voice_handlers.go:28 +0x294
...

Test plan

• go build ./... in disgo and a downstream voice-bot consumer
• Downstream integration tests that previously SIGSEGV'd within ~30s now run to completion