Return if no previousSibling of el found

Cater for scenario where there is no previous sibling of el. Solution to #1249 and similar to #982. Occurs in scenario where dragging between lists using a library such as RubaXa Sortable.

src/directives/public/for.js

@@ -539,6 +539,7 @@ function findPrevFrag (frag, anchor, id) {
     el !== anchor
   ) {
     el = el.previousSibling
+    if (!el) return
     frag = el.__vfrag__
   }
   return frag