interrupt corner case - panic: runtime error:  close of closed channel

This happens when an interrupt is taking time and a second interrupt is triggered:

    ^C^C2014/08/13 19:26:55 Actor on localhost:18557 shutdown successfully
    2014/08/13 19:26:55 Received SIGINT (Ctrl+C).  Shutting down...
    2014/08/13 19:26:55 Miner already shutdown
    panic: runtime error: close of closed channel

    goroutine 27 [running]:
    runtime.panic(0x83b22e0, 0x85fe355)
            /usr/local/go/src/pkg/runtime/panic.c:279 +0xe9
    main.func·011()
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcsim/sim.go:195 +0x41
    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