Fix handleTouchEnd called not after a handleTouchStart

[serut]

In my application, when I click the first time inside the scrollbar with a Chrome under mobile emulation, the event handleTouchEnd is called but no handleTouchStart.
This PR protects the handleTouchEnd event handler to throw that exception, since this.eventPreviousValues is undefined:

Uncaught TypeError: Cannot read property 'deltaX' of undefined
    at t.value (/bundle.js?5a5c0ac8dc6a61225da1:2:1002364)
    at Object.j (/bundle.js?5a5c0ac8dc6a61225da1:2:762488)
    at Object.invokeGuardedCallback (/bundle.js?5a5c0ac8dc6a61225da1:2:761767)
    at Object.invokeGuardedCallbackAndCatchFirstError (/bundle.js?5a5c0ac8dc6a61225da1:2:761882)
    at Z (/bundle.js?5a5c0ac8dc6a61225da1:2:763535)
    at ee (bundle.js?5a5c0ac8dc6a61225da1:2:763990)
    at ne (/bundle.js?5a5c0ac8dc6a61225da1:2:764175)
    at Array.forEach (native)
    at J (/bundle.js?5a5c0ac8dc6a61225da1:2:763804)
    at le (/bundle.js?5a5c0ac8dc6a61225da1:2:764909)

[serut]

I thought at first one of my issue came from your lib, so I fix it. But in fact it was something else.

Anyway, you should merge it to prevent people using Chrome debugger to see that issue and loose some time on it as that event can be safely ignored.