refactor(picker): selectedIndex computed logic

src/components/picker/picker.vue

@@ -236,12 +236,12 @@
             useTransition: USE_TRANSITION
           })
           wheel.on('scrollEnd', () => {
-            const y = wheel.y
             let selectedIndex
             if (USE_TRANSITION) {
               selectedIndex = wheel.getSelectedIndex()
             } else {
               // fixed BScroll not calculating selectedIndex when setting useTransition to false
+              const y = wheel.y
               if (y > wheel.minScrollY) {
                 selectedIndex = 0
               } else if (y < wheel.maxScrollY) {