fix a crash risk

rtsp/rtsp-client.go

@@ -235,6 +235,8 @@ func (client *RTSPClient) requestStream(timeout time.Duration) (err error) {
 	headers = make(map[string]string)
 	headers["Accept"] = "application/sdp"
 	resp, err = client.Request("DESCRIBE", headers)
+	if err != nil {
+		if resp != nil {
 			authorization, _ := client.checkAuth("DESCRIBE", resp)
 			if len(authorization) > 0 {
 				headers := make(map[string]string)
@@ -245,7 +247,10 @@ func (client *RTSPClient) requestStream(timeout time.Duration) (err error) {
 			if err != nil {
 				return err
 			}
-
+		} else {
+			return err
+		}
+	}
 	_sdp, err := sdp.ParseString(resp.Body)
 	if err != nil {
 		return err