handle null pointers: cJSON_AddItemToObjectCS

cJSON.c

@@ -1826,7 +1826,7 @@ CJSON_PUBLIC(void) cJSON_AddItemToObject(cJSON *object, const char *string, cJSO
 /* Add an item to an object with constant string as key */
 CJSON_PUBLIC(void) cJSON_AddItemToObjectCS(cJSON *object, const char *string, cJSON *item)
 {
-    if (!item)
+    if ((item == NULL) || (string == NULL))
     {
         return;
     }