fix: Add check before update

[ishmalez]

Issue

Details

Sometimes when mounting and unmounting different instances of Flicking slider, variable this.slotDiffer is undefined, throwing an exception on beforeUpdate() execution.
This fix is just to avoid the exception.

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[malangfox]

LGTM

[RodrigoProjects]

Thanks for the fix! Will this be in the next minor @malangfox ?