Exit gracefully when no speakers are configured

[hilli]

Since that doesn't happen now...

❯ kefw2 status
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x2cb61c]

goroutine 1 [running]:
github.com/hilli/go-kef-w2/kefw2.(*KEFSpeaker).Source(0x40000db848?)
	/home/hilli/github.com/hilli/go-kef-w2/kefw2/kefw2.go:143 +0x1c
github.com/hilli/go-kef-w2/cmd/kefw2/cmd.glob..func13(0x780380?, {0x7bf730?, 0x0?, 0x0?})
	/home/hilli/github.com/hilli/go-kef-w2/cmd/kefw2/cmd/status.go:19 +0x2c
github.com/spf13/cobra.(*Command).execute(0x780380, {0x7bf730, 0x0, 0x0})
	/home/hilli/.go/pkg/mod/github.com/spf13/[email protected]/command.go:944 +0x5ac
github.com/spf13/cobra.(*Command).ExecuteC(0x77fae0)
	/home/hilli/.go/pkg/mod/github.com/spf13/[email protected]/command.go:1068 +0x340
github.com/spf13/cobra.(*Command).Execute(...)
	/home/hilli/.go/pkg/mod/github.com/spf13/[email protected]/command.go:992
github.com/hilli/go-kef-w2/cmd/kefw2/cmd.Execute()
	/home/hilli/github.com/hilli/go-kef-w2/cmd/kefw2/cmd/root.go:65 +0x24
main.main()
	/home/hilli/github.com/hilli/go-kef-w2/cmd/kefw2/kefw2.go:27 +0x1c
exit status 2