Fix nextTreeData

bintree.c

@@ -77,12 +77,14 @@ void *nextTreeData(TreeNode *root)
         current = root;
     }
 
-    while (current != NULL)
-    {
+    while (current != NULL){
         stack = push(stack, current);
         current = current->left;
     }
 
+    if(stack == NULL){
+        return 0;
+    }
     TreeNode *node = (TreeNode *)top(stack);
     stack = pop(stack);