added missing onready allocation for subclasses
Created by: leezh
Fixes https://github.com/godotengine/godot/issues/3158
The problem was with https://github.com/godotengine/godot/commit/30c12297dc6df7d35df140475c0cec7308aea77a. While gd_parser allocated the container for p_class->ready in the main classes, it didn't do so for subclasses.