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>

bundle-uri.c

@@ -532,12 +532,14 @@ static int fetch_bundles_by_token(struct repository *r,
 	 */
 	if (!repo_config_get_value(r,
 				   "fetch.bundlecreationtoken",
-				   &creationTokenStr) &&
-	    sscanf(creationTokenStr, "%"PRIu64, &maxCreationToken) == 1 &&
-	    bundles.items[0]->creationToken <= maxCreationToken) {
+				   &creationTokenStr)) {
+		if (sscanf(creationTokenStr, "%"PRIu64, &maxCreationToken) != 1)
+			maxCreationToken = 0;
+		if (bundles.items[0]->creationToken <= maxCreationToken) {
 			free(bundles.items);
 			return 0;
 		}
+	}
 
 	/*
 	 * Attempt to download and unbundle the minimum number of bundles by