gpgsm: Possible improvement for some rare P12 files.

* sm/minip12.c (parse_shrouded_key_bag): Increase size of salt buffer.

Reported on the mailing list. The change does not seem to have a big
regression risk, thus applied. See below for the mail

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https://lists.gnupg.org/pipermail/gnupg-users/2024-September/067312.html