Panic when using JSON stdout metric output

[na--]

When executing even a very simple script (k6 run -o json samples/http_get.js) with the latest master code, the following panic happens:

...
{"type":"Metric","data":{"name":"vus_max","type":"gauge","contains":"default","tainted":null,"thresholds":[],"submetrics":null,"sub":{"name":"","parent":"","suffix":"","tags":null}},"metric":"vus_max"}
{"type":"Point","data":{"time":"2018-07-17T12:59:17.321885342+03:00","value":1,"tags":null},"metric":"vus_max"}
panic: write /dev/stdout: file already closed

goroutine 1 [running]:
github.com/loadimpact/k6/cmd.fprintf(0xe7a560, 0xc421d260a0, 0xd05fe4, 0x7, 0xc420c0bce0, 0x1, 0x1, 0x50)
	/home/go/src/github.com/loadimpact/k6/cmd/root.go:116 +0xf0
github.com/loadimpact/k6/cmd.glob..func11(0x14c4220, 0xc4204249c0, 0x1, 0x3, 0x0, 0x0)
	/home/go/src/github.com/loadimpact/k6/cmd/run.go:400 +0x332e
github.com/loadimpact/k6/vendor/github.com/spf13/cobra.(*Command).execute(0x14c4220, 0xc420424930, 0x3, 0x3, 0x14c4220, 0xc420424930)
	/home/go/src/github.com/loadimpact/k6/vendor/github.com/spf13/cobra/command.go:698 +0x46d
github.com/loadimpact/k6/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0x14c4000, 0x52, 0xc42000c1a0, 0xc420458000)
	/home/go/src/github.com/loadimpact/k6/vendor/github.com/spf13/cobra/command.go:783 +0x2e4
github.com/loadimpact/k6/vendor/github.com/spf13/cobra.(*Command).Execute(0x14c4000, 0xc420089f78, 0x405d8c)
	/home/go/src/github.com/loadimpact/k6/vendor/github.com/spf13/cobra/command.go:736 +0x2b
github.com/loadimpact/k6/cmd.Execute()
	/home/go/src/github.com/loadimpact/k6/cmd/root.go:87 +0x31
main.main()
	/home/go/src/github.com/loadimpact/k6/main.go:28 +0x20

[na--]

The panic seems to be the result of the fixes I did last week to comply with the latest errcheck changes, but that just means that the issue was there before (trying to write to a closed stdout), we just silently ignored it.