maps should be made before being unmarshalled into

This code crashes assigning to a nil map. It should probably not.

 package main

 import (
  "fmt"
  "launchpad.net/goyaml"
 )

 func main() {
  var x map[string]string
  if err := goyaml.Unmarshal([]byte("foo: y\n"), &x); err != nil {
   fmt.Printf("unmarshal error: %v", err)
  } else {
   fmt.Printf("%#v\n", x)
  }
 }