saslContinue crashes in proxy mode

[AlekSi]

FerretDB version

0aad83a

Backend

Any

Environment

Any

What did you do?

1. task env-up
2. task run-proxy-secured
3. docker compose exec mongodb mongosh 'mongodb://username:password@host.docker.internal:27018/?tls=true&tlsCertificateKeyFile=/etc/certs/client.pem&tlsCaFile=/etc/certs/rootCA-cert.pem'

What did you expect to see?

FerretDB not crashing.

What did you see instead?

2024-02-25T09:57:05.181+0400	DPANIC	// 127.0.0.1:49990 -> 127.0.0.1:27018 	clientconn/conn.go:166	runtime error: invalid memory address or nil pointer dereference
(err = <nil>)
github.com/FerretDB/FerretDB/internal/clientconn.(*conn).run.func3
	/Users/aleksi/Code/FerretDB/FerretDB/internal/clientconn/conn.go:166
runtime.gopanic
	/opt/homebrew/Cellar/go/1.22.0/libexec/src/runtime/panic.go:770
runtime.panicmem
	/opt/homebrew/Cellar/go/1.22.0/libexec/src/runtime/panic.go:261
runtime.sigpanic
	/opt/homebrew/Cellar/go/1.22.0/libexec/src/runtime/signal_unix.go:881
github.com/xdg-go/scram.(*ServerConversation).Step
	/Users/aleksi/Soft/GOPATH/pkg/mod/github.com/xdg-go/scram@v1.1.2/server_conv.go:47
github.com/FerretDB/FerretDB/internal/handler.(*Handler).MsgSASLContinue
	/Users/aleksi/Code/FerretDB/FerretDB/internal/handler/msg_saslcontinue.go:47
github.com/FerretDB/FerretDB/internal/clientconn.(*conn).handleOpMsg
	/Users/aleksi/Code/FerretDB/FerretDB/internal/clientconn/conn.go:571
github.com/FerretDB/FerretDB/internal/clientconn.(*conn).route
	/Users/aleksi/Code/FerretDB/FerretDB/internal/clientconn/conn.go:428
github.com/FerretDB/FerretDB/internal/clientconn.(*conn).run
	/Users/aleksi/Code/FerretDB/FerretDB/internal/clientconn/conn.go:312
github.com/FerretDB/FerretDB/internal/clientconn.acceptLoop.func1
	/Users/aleksi/Code/FerretDB/FerretDB/internal/clientconn/listener.go:305
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
(err = <nil>)