Out of bounds error

[weihao]

When you move the cursor out of bound this error occurs.

client.js?06a0:102 TypeError: Cannot read property '0' of undefined
    at VueComponent.onMouseMove (vue-image-compare.js?d322:223)
    at mousemove (vue-image-compare.js?d322:355)
    at invokeWithErrorHandling (vue.runtime.esm.js?2b0e:1854)
    at HTMLElement.invoker (vue.runtime.esm.js?2b0e:2179)
    at HTMLElement.original._wrapper (vue.runtime.esm.js?2b0e:6917)
_callee$ @ client.js?06a0:102
tryCatch @ runtime.js?96cf:63
invoke @ runtime.js?96cf:293
eval @ runtime.js?96cf:118
asyncGeneratorStep @ asyncToGenerator.js?1da1:3
_next @ asyncToGenerator.js?1da1:25
eval @ asyncToGenerator.js?1da1:32
eval @ asyncToGenerator.js?1da1:21
eval @ client.js?06a0:64
globalHandleError @ vue.runtime.esm.js?2b0e:1870
handleError @ vue.runtime.esm.js?2b0e:1839
invokeWithErrorHandling @ vue.runtime.esm.js?2b0e:1862
invoker @ vue.runtime.esm.js?2b0e:2179
original._wrapper @ vue.runtime.esm.js?2b0e:6917

[weihao]

#10 has the problem summary and a fix.

[weihao]

@marcincichocki Could you review my MR on #10