bundle-uri: avoid using undefined output of `sscanf()`

bundle-uri: avoid using undefined output of `sscanf()`

In c429bed102 (bundle-uri: store fetch.bundleCreationToken, 2023-01-31)
code was introduced that assumes that an `sscanf()` call leaves its
output variables unchanged unless the return value indicates success.

However, the POSIX documentation makes no such guarantee:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/sscanf.html

So let's make sure that the output variable `maxCreationToken` is
always well-defined.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>