[BUG] [Dafang] An error occurs while making snapshot request
**Describe the bug**
Installed the plugin, can see the camera in Home app, however I don't see a picture and receive an error in Homebridge.

[Dafang] An error occurs while making stream request
[2018-12-28 20:11:14] [Dafang] ERROR: FFmpeg exited with code -2
[2018-12-28 20:11:17] [Dafang] Snapshot from My Dafang at 480x270
[2018-12-28 20:11:17] [Dafang] An error occurs while making snapshot request

**Expected behavior**
I was expecting to see a snapshot and when pressing a live image.

**Config**
"platform": "Dafang",
            "mqtt": {
                "hostBroker": true,
                "port": 1883,
                "host": "192.168.1.35",
                "debug": true,
                "mongoUrl": "mongodb://localhost:32768/mqtt"
            },
            "cameras": [
                {
                    "cameraName": "My Dafang",
                    "cameraRTSPStreamUrl": "rtsp://192.168.1.49:8554/unicast",
                    "mqttTopic": "home/Dafang/#",
                    "folder": "/volume1/homes/User/Snapshots Dafang/"

**Logs**
[Dafang] An error occurs while making stream request
[2018-12-28 20:11:14] [Dafang] ERROR: FFmpeg exited with code -2
[2018-12-28 20:11:17] [Dafang] Snapshot from My Dafang at 480x270
[2018-12-28 20:11:17] [Dafang] An error occurs while making snapshot request

**Desktop (please complete the following information):**
 - OS: Homebridge Docker on Synology
 - Node Version  v8