interrupt corner case - panic

when btcsim is interrupted just after it starts up, there's a chance of hitting this:

    2014/08/18 21:28:57 Received SIGINT (Ctrl+C).  Shutting down...
    21:28:57 2014-08-18 [INF] BTCW: Received SIGINT (Ctrl+C).  Shutting down...
    21:28:57 2014-08-18 [WRN] BTCW: Server shutting down
    21:28:57 2014-08-18 [INF] BTCW: Shutdown complete
    panic: runtime error: invalid memory address or nil pointer dereference
    [signal 0xb code=0x1 addr=0x1 pc=0x80df4f7]

    goroutine 28 [running]:
    runtime.panic(0x83b2bc0, 0x85ff4b3)
            /usr/local/go/src/pkg/runtime/panic.c:279 +0xe9
    github.com/conformal/btcrpcclient.(*Client).Shutdown(0x0)
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcrpcclient/infrastructure.go:938 +0x1a7
    main.(*Actor).Stop(0x189a2360)
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcsim/actor.go:341 +0x73
    main.Close(0x1885a540, 0x1, 0x1)
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcsim/sim.go:275 +0x60
    main.func·015()
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcsim/sim.go:239 +0x80
    main.mainInterruptHandler()
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcsim/signal.go:47 +0xbf
    created by main.addInterruptHandler
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcsim/signal.go:64 +0xb6