etag argument check

[Francesco Iannuzzelli]

I have experienced a few process crashes with the following error, it seems to happen randomly with the same (cached) image being served ok at times, and some other times with an error

[2018-11-14T09:59:11.435Z] ERROR: dadi-cdn/22192 on ip-172-31-17-37: argument entity must be string, Buffer, or fs.Stats
    TypeError: argument entity must be string, Buffer, or fs.Stats
        at etag (/data/cdn/node_modules/etag/index.js:83:11)
        at workQueue.run.then (/data/cdn/node_modules/@dadi/cdn/dadi/lib/controller/index.js:77:24)
        at <anonymous>
        at process._tickDomainCallback (internal/process/next_tick.js:229:7)

https://github.com/dadi/cdn/blob/develop/dadi/lib/controller/index.js#L77

CDN 3.4.1