Fix SAM-2 preauth when password argument is used

sam2_process accesses gak_data to get the password, so that it can do
string-to-key with the etype in the SAM-2 challenge.  When #7642
changed gic_pwd.c to use struct gak_password instead of krb5_data,
sam2_process wasn't altered to match.  We don't see a problem when the
password is read through the prompter (as with kinit), because the
password winds up in the storage field at the beginning of the
gak_password structure.  But when a password is supplied as a
parameter (as with ksu), the storage field is empty and we get the
wrong answer from sam2_process.

ticket: 7862
target_version: 1.12.2
tags: pullup