Assert fp is not None

Tests/test_file_cur.py

@@ -26,6 +26,7 @@ def test_invalid_file() -> None:
     no_cursors_file = "Tests/images/no_cursors.cur"
 
     cur = CurImagePlugin.CurImageFile(TEST_FILE)
+    assert cur.fp is not None
     cur.fp.close()
     with open(no_cursors_file, "rb") as cur.fp:
         with pytest.raises(TypeError):

src/PIL/CurImagePlugin.py

@@ -38,6 +38,7 @@ class CurImageFile(BmpImagePlugin.BmpImageFile):
     format_description = "Windows Cursor"
 
     def _open(self) -> None:
+        assert self.fp is not None
         offset = self.fp.tell()
 
         # check magic