fix(RecycleScroller): gridItems is undefined when scrollToItem, fix #773#868

TheRealMattV wants to merge 1 commit intoAkryum:v1Akryum/vue-virtual-scroller:v1from

TheRealMattV:bugfix/griditems_undefinedTheRealMattV/vue-virtual-scroller:bugfix/griditems_undefinedCopy head branch name to clipboard