Fix segfault in Server.Serve

[nklaassen]

This PR fixes a segfault that can occur in lib/srv/regular.(*Server).Serve.

I discovered this while running TestHSMRotation on a ubuntu ec2 instance, it seems to reproduce in 1/5 runs. The segfault occurs when Server.close() is called before Server.Serve.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x3c49714]

goroutine 5961 [running]:
github.com/gravitational/teleport/lib/srv/regular.(*Server).Serve(0xc001f6b080, {0x546e810, 0xc001f5bad0})
        /home/ubuntu/src/teleport2/lib/srv/regular/sshserver.go:388 +0x134
created by github.com/gravitational/teleport/lib/service.(*TeleportProcess).initProxyEndpoint.func6
        /home/ubuntu/src/teleport2/lib/service/service.go:3506 +0x2be
FAIL    github.com/gravitational/teleport/integration/hsm       13.760s
FAIL

[github-actions]

@nklaassen See the table below for backport results.

Branch	Result
branch/v10	Create PR
branch/v7	Failed
branch/v8	Failed
branch/v9	Failed