decode_signed_value raises ValueError for authenticated malformed timestamps

   `decode_signed_value` raises `ValueError` for a syntactically malformed
   timestamp after the cookie signature has been successfully verified.

   I expected an invalid signed cookie to be rejected with `None`, consistent
   with `RequestHandler.get_signed_cookie`'s documented return behavior, rather
   than causing an exception in request handling.

   ## Reproducer

   This example constructs a structurally valid v2 signed value whose timestamp
   field is `b"a"` and computes a valid HMAC using the configured secret:

   ```python
   from tornado.escape import utf8
   from tornado.web import _create_signature_v2, decode_signed_value

   def field(value: bytes) -> bytes:
       return str(len(value)).encode("ascii") + b":" + value

   secret = b"s"
   name = "a"

   prefix = b"|".join(
       [
           b"2",
           field(b"0"),
           field(b"a"),       # non-decimal timestamp
           field(utf8(name)),
           field(b""),
           b"",
       ]
   )
   cookie = prefix + _create_signature_v2(secret, prefix)

   assert decode_signed_value(
       secret, name, cookie, clock=lambda: 1_500_000_000
   ) is None

  ValueError: invalid literal for int() with base 10: b'a'
The exception is raised by:

```python
  timestamp = int(timestamp_bytes)