map[string]struct{} unused variable

[jhwz]

If there's a map[string]struct{} field in a struct, the generated code will not compile

Eg:

type Example struct {
	M map[string]struct{}
}

produces

// EncodeMsg implements msgp.Encodable
func (z *Example) EncodeMsg(en *msgp.Writer) (err error) {
	// map header, size 1
	// write "M"
	err = en.Append(0x81, 0xa1, 0x4d)
	if err != nil {
		return
	}
	err = en.WriteMapHeader(uint32(len(z.M)))
	if err != nil {
		err = msgp.WrapError(err, "M")
		return
	}
	for za0001, za0002 := range z.M {
		err = en.WriteString(za0001)
		if err != nil {
			err = msgp.WrapError(err, "M")
			return
		}
		// map header, size 0
		err = en.Append(0x80)
		if err != nil {
			return
		}
	}
	return
}

where za0002 is unused and causes the code to not compile