[scala/scala#11153] add null-check in ConcatIterator.advance

Author: tIGO

library/src/scala/collection/Iterator.scala

@@ -1050,7 +1050,7 @@ object Iterator extends IterableFactory[Iterator] {
         tail = tail.tail
         merge()
         if (currentHasNextChecked) true
-        else if (current.hasNext) {
+        else if ((current ne null) && current.hasNext) {
           currentHasNextChecked = true
           true
         } else advance()