Fix self.decode typo (#9445)

Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>

Author: bysiber

src/PIL/ImageFile.py

@@ -579,10 +579,7 @@ def feed(self, data: bytes) -> None:
                 pass  # not enough data
             else:
                 flag = hasattr(im, "load_seek") or hasattr(im, "load_read")
-                if flag or len(im.tile) != 1:
-                    # custom load code, or multiple tiles
-                    self.decode = None
-                else:
+                if not flag and len(im.tile) == 1:
                     # initialize decoder
                     im.load_prepare()
                     d, e, o, a = im.tile[0]