Segfault when running ```headscale nodes list --output json``` when backend is not running.

[rcmcdonald91]

When running headscale nodes list --output json against a server that is offline, a segfault is encountered. headscale nodes list fails gracefully with Could not connect..... This segfault is problematic for frontends that consume json output exclusively.

$ headscale nodes list --output json
panic: runtime error: invalid memory address or nil pointer dereference
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0xb1feab]

goroutine 1 [running]:
google.golang.org/grpc.(*ClientConn).Close(0x0)
        google.golang.org/grpc@v1.46.0/clientconn.go:998 +0x4b
panic({0x1124f80, 0x1c52bc0})
        runtime/panic.go:838 +0x207
google.golang.org/grpc.(*ClientConn).Invoke(0x80203cf18?, {0x1464490?, 0xc00054ec60?}, {0x1292a76?, 0x0?}, {0x118afc0?, 0xc00054c040?}, {0x118b080?, 0xc00054c180?}, {0x0, ...})
        google.golang.org/grpc@v1.46.0/call.go:32 +0x5e
github.com/juanfont/headscale/gen/go/headscale/v1.(*headscaleServiceClient).ListMachines(0xc00007a780, {0x1464490, 0xc00054ec60}, 0xc0002f4420?, {0x0, 0x0, 0x0})
        github.com/juanfont/headscale/gen/go/headscale/v1/headscale_grpc.pb.go:195 +0xce
github.com/juanfont/headscale/cmd/headscale/cli.glob..func16(0x1c666e0?, {0x125c23c?, 0x2?, 0x2?})
        github.com/juanfont/headscale/cmd/headscale/cli/nodes.go:168 +0x2d0
github.com/spf13/cobra.(*Command).execute(0x1c666e0, {0xc000550980, 0x2, 0x2})
        github.com/spf13/cobra@v1.4.0/command.go:860 +0x663
github.com/spf13/cobra.(*Command).ExecuteC(0x1c68260)
        github.com/spf13/cobra@v1.4.0/command.go:974 +0x3b4
github.com/spf13/cobra.(*Command).Execute(...)
        github.com/spf13/cobra@v1.4.0/command.go:902
github.com/juanfont/headscale/cmd/headscale/cli.Execute()
        github.com/juanfont/headscale/cmd/headscale/cli/root.go:85 +0x25
main.main()
        github.com/juanfont/headscale/cmd/headscale/headscale.go:42 +0x235

To Reproduce

1. Invoke headscale with --output json against a backend that is not running.

Context info

• Version of headscale used: v0.16.0-beta4 (and v0.15.0)
• Version of tailscale client: N/A
• OS (e.g. Linux, Mac, Cygwin, WSL, etc.) and version: FreeBSD 13.1-RELEASE

[luckman212]

@theonemcdonald Does this mean headscale is coming someday to pfSense? 👀