Invalid custom plugin output target is not handled correctly

[atc0005]

I spotted this while working on #264:

go-nagios/nagios.go

Lines 398 to 407 in 55589e5

	// SetOutputTarget assigns a target for Nagios plugin output. By default
	// output is emitted to os.Stdout.
	func (p *Plugin) SetOutputTarget(w io.Writer) {
	// Guard against potential nil argument.
	if w == nil {
	p.outputSink = os.Stdout
	}
	p.outputSink = w
	}

The nil handling check should return early to prevent the custom (invalid) value from being used as the output target.