prevent calling function on undefined fileInstance

[PhilWaldmann]

getFile() could potentially return an empty object. Calling e.g. updateStatus on that object will throw an error.
I observed this behavior on Firefox. E.g. the thumbnailevent-handler was invoked after the this.files was cleared. It resulted in:

TypeError: this.getFile(...).updateDataUrl is not a function
  at this(~/vue-clip/dist/vue-clip.js:992:0)
  at fn(~/vue/dist/vue.esm.js:163:0)
  at callback(~/vue-clip/dist/vue-clip.js:1836:0)
  at _this(~/vue-clip/dist/vue-clip.js:2902:0)

Firefox 54 and 52
vue-clip 1.0.0
vue 2.3.3