remove loitering bug in BTree.java

src/main/java/edu/princeton/cs/algs4/BTree.java

@@ -67,7 +67,7 @@ private Node(int k) {
     // external nodes: only use key and value
     private static class Entry {
         private Comparable key;
-        private final Object val;
+        private Object val;
         private Node next;     // helper field to iterate over array entries
         public Entry(Comparable key, Object val, Node next) {
             this.key  = key;
@@ -184,6 +184,7 @@ private Node insert(Node h, Key key, Value val, int ht) {
                     Node u = insert(h.children[j++].next, key, val, ht-1);
                     if (u == null) return null;
                     t.key = u.children[0].key;
+                    t.val = null;
                     t.next = u;
                     break;
                 }