Loading images from a URL fails if the image file is empty

[fra967]

Consider this URL
http://media.thesimplygroup.com/images/236220/1/1.jpg
http code 200
empty file (0 bytes)

CDN crashes with

/cdn/node_modules/@dadi/cdn/dadi/lib/handlers/image.js:308
      require('lwip').open(imageBuffer, imageInfo.format, (err, image) => {
                                                 ^
TypeError: Cannot read property 'format' of undefined
    at processImage (/cdn/node_modules/@dadi/cdn/dadi/lib/handlers/image.js:308:50)

[mingard]

Fixed in #248