panic in checkpoint code

[rade]

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x58 pc=0x4088cc]

goroutine 18 [running]:
main.appMain.func1(0x0, 0x7efcab814868, 0xc821a2ddd0)
        /go/src/github.com/weaveworks/scope/prog/app.go:56 +0x1c
github.com/weaveworks/scope/vendor/github.com/weaveworks/go-checkpoint.CheckInterval.func1(0x13a52453c000, 0xc820273100, 0x198b590, 0xc82034e540)
        /go/src/github.com/weaveworks/scope/vendor/github.com/weaveworks/go-checkpoint/checkpoint.go:208 +0xe7
created by github.com/weaveworks/scope/vendor/github.com/weaveworks/go-checkpoint.CheckInterval
        /go/src/github.com/weaveworks/scope/vendor/github.com/weaveworks/go-checkpoint/checkpoint.go:213 +0xb7

The issue here is that the checkpoint callback can get invoked with a nil first argument and error second argument when something went wrong, yet we pay no attention to the error and immediately dereference the first arg.