Avoid uninitialized data in t_prf.c

Avoid uninitialized data in t_prf.c

In t_prf.c, make sure that the partially initialized, faked-up
structures gss_union_ctx_id_desc and krb5_gss_ctx_id_rec are zeroed.
This avoids uninitialized reads in gss_pseudo_random(), which can
cause intermittent test failures on some platforms.

(cherry picked from commit b5143bdc766ad4819355ac2cd52f685ec5dafa16)

ticket: 8072
version_fixed: 1.13.1
status: resolved