Add availability check in Image methods

Author: kojix2

src/uing/image.cr

@@ -10,6 +10,7 @@ module UIng
     end
 
     def append(pixels, pixel_width : Int32, pixel_height : Int32, byte_stride : Int32) : Nil
+      check_available
       LibUI.image_append(@ref_ptr, pixels, pixel_width, pixel_height, byte_stride)
     end
 
@@ -19,7 +20,12 @@ module UIng
       @released = true
     end
 
+    private def check_available : Nil
+      raise "Image has already been released" if @released
+    end
+
     def to_unsafe
+      check_available
       @ref_ptr
     end
   end