Adds json_object_get_value_at function to access values in objects in O(1) time.

Also adds some missing null checking.

parson.c

@@ -983,14 +983,20 @@ size_t json_object_get_count(const JSON_Object *object) {
 }
 
 const char * json_object_get_name(const JSON_Object *object, size_t index) {
-    if (index >= json_object_get_count(object))
+    if (object == NULL || index >= json_object_get_count(object))
         return NULL;
     return object->names[index];
 }
 
+JSON_Value * json_object_get_value_at(const JSON_Object *object, size_t index) {
+    if (object == NULL || index >= json_object_get_count(object))
+        return NULL;
+    return object->values[index];
+}
+
 /* JSON Array API */
 JSON_Value * json_array_get_value(const JSON_Array *array, size_t index) {
-    if (index >= json_array_get_count(array))
+    if (array == NULL || index >= json_array_get_count(array))
         return NULL;
     return array->items[index];
 }

parson.h

@@ -127,6 +127,7 @@ int           json_object_dotget_boolean(const JSON_Object *object, const char *
 /* Functions to get available names */
 size_t        json_object_get_count(const JSON_Object *object);
 const char  * json_object_get_name (const JSON_Object *object, size_t index);
+JSON_Value  * json_object_get_value_at(const JSON_Object *object, size_t index);
     
 /* Creates new name-value pair or frees and replaces old value with a new one. 
  * json_object_set_value does not copy passed value so it shouldn't be freed afterwards. */