Update inorder-successor-in-bst.cpp

Author: kamyu104

C++/inorder-successor-in-bst.cpp

@@ -14,7 +14,7 @@ class Solution {
 public:
     TreeNode* inorderSuccessor(TreeNode* root, TreeNode* p) {
         // If it has right subtree.
-        if (p->right) {
+        if (p && p->right) {
             p = p->right;
             while (p->left) {
                 p = p->left;