ffmpeg keeps running even though no one is watching
It seems that `ffmpeg` doesn't close when I close the Home app on my device.

In this screenshot, there's absolutely no one watching the stream. 

<img width="762" alt="Screenshot 2020-03-24 15 37 32" src="https://user-images.githubusercontent.com/19761269/77413683-c0ee4300-6de5-11ea-87d6-9d1d306398a3.png">

Homebridge config:
```
{
    "bridge": {
        "name": "Homebridge",
        "username": "0E:47:CF:4D:B9:66",
        "port": 51600,
        "pin": "840-25-461"
    },
    "accessories": [],
    "platforms": [
        {
            "name": "Config",
            "port": 7778,
            "auth": "none",
            "theme": "deep-purple",
            "tempUnits": "c",
            "lang": "auto",
            "platform": "config"
        },
        {
            "name": "Camera ffmpeg",
            "cameras": [
                {
                    "name": "Front Gate",
                    "manufacturer": "D-Link",
                    "model": "DCS-933L",
                    "serialNumber": "YET-TO-SPECIFY",
                    "firmwareRevision": "1.15.01",
                    "videoConfig": {
                        "source": "-re -f mjpeg -i http://admin:xxxxx@10.0.0.6/mjpeg.cgi",
                        "stillImageSource": "-f mjpeg -i http://admin:xxxxx@10.0.0.6/image.jpg",
                        "maxWidth": 1280,
                        "maxHeight": 720,
                        "maxFPS": 20,
                        "vcodec": "libx264",
                        "debug": false
                    }
                }
            ],
            "platform": "Camera-ffmpeg"
        }
    ]
}
```
