fix(sticky): improve onScroll logic

resolve  possible RTE with `self.current`

src/components/sticky/sticky.js

@@ -204,7 +204,7 @@ function MdSticky($document, $mdConstant, $$rAF, $mdUtil) {
  }
 
  // If the next item is close to the current one, pull the current one down into view
- if (self.next && scrollTop >= self.next.top - self.current.height) {
+ if (self.next && self.current && (scrollTop >= (self.next.top - self.current.height))) {
  translate(self.current, scrollTop + (self.next.top - scrollTop - self.current.height));
  return;
  }