buffer is not allocated in client.Read function

[ozanh]

Hello,
Thank you for the S7 library. I'd like to report a bug. In the client.Read function, buffer is not allocated in line 383 as below, which causes panic while copying response.Data in client.readArea. I am not sure if it is wise to make var buffer = make([]byte, 255) in client.Read function or changing function signature to pass buffer as a parameter.

gos7/client.go

Lines 374 to 384 in a789e2a

	func (mb *client) Read(variable string) (value interface{}, err error) {
	variable = strings.ToUpper(variable) //upper
	variable = strings.Replace(variable, " ", "", -1) //remove spaces
	if variable == "" {
	err = fmt.Errorf("input variable is empty, variable should be S7 syntax")
	return
	}
	//var area, dbNumber, start, amount, wordLen int
	var buffer []byte
	switch valueArea := variable[0:2]; valueArea {