Properly defer a client cleanup in case of a pnic

command/build/command.go

@@ -5,6 +5,7 @@ import "github.com/mitchellh/packer/packer"
 type Command byte
 
 func (Command) Run(env packer.Environment, arg []string) int {
+	env.Ui().Say("YO!")
 	return 0
 }
 

packer.go

@@ -11,6 +11,8 @@ import (
 )
 
 func main() {
+	defer plugin.CleanupClients()
+
 	commands := map[string]string {
 		"build": "packer-build",
 	}