gpgsm: Skip the optional PKCS#12 PBES2 keyLength parameter

* sm/minip12.c (parse_bag_encrypted_data): Parse and ignore the
  optional keyLength.
(parse_shrouded_key_bag): Ditto.

We have noticed newer German Telekom issued certificates which for the
first time include this optional parameter. This patch allows to
properly parse them. See RFC-2898, A2 for the specs.