Reset key-generation parameters for each enctype

In add_key_pwd, initialize s2k_params to NULL inside the loop over
enctypes instead of outside the loop, so that if the afs3 salt type
is used it does not contaminate later enctype/salt pairs in the list.

(cherry picked from commit 381cf01afb13ad28de0927de37f8e1d12749bf49)

ticket: 7733
version_fixed: 1.12
status: resolved