Handle disposing GIF background from outside palette

Tests/test_file_gif.py

@@ -1142,6 +1142,12 @@ def test_rgba_transparency(tmp_path):
         assert_image_equal(hopper("P").convert("RGB"), reloaded)
 
 
+def test_background_outside_palettte(tmp_path):
+    with Image.open("Tests/images/background_outside_palette.gif") as im:
+        im.seek(1)
+        assert im.info["background"] == 255
+
+
 def test_bbox(tmp_path):
     out = str(tmp_path / "temp.gif")
 

src/PIL/GifImagePlugin.py

@@ -335,6 +335,8 @@ def _seek(self, frame, update_image=True):
 
         def _rgb(color):
             if self._frame_palette:
+                if color * 3 + 3 > len(self._frame_palette.palette):
+                    color = 0
                 color = tuple(self._frame_palette.palette[color * 3 : color * 3 + 3])
             else:
                 color = (color, color, color)