Ensure image is loaded in remap_palette

Tests/test_image.py

@@ -582,6 +582,10 @@ class TestImage:
         assert ext_individual == ext_multiple
 
     def test_remap_palette(self):
+        # Test identity transform
+        with Image.open("Tests/images/hopper.gif") as im:
+            assert_image_equal(im, im.remap_palette(list(range(256))))
+
         # Test illegal image mode
         with hopper() as im:
             with pytest.raises(ValueError):

src/PIL/Image.py

@@ -1796,6 +1796,7 @@ class Image:
 
         if source_palette is None:
             if self.mode == "P":
+                self.load()
                 real_source_palette = self.im.getpalette("RGB")[:768]
             else:  # L-mode
                 real_source_palette = bytearray(i // 3 for i in range(768))