[krbdev.mit.edu #6796] segfault due to uninitialized variable in S4U

[Arlene Berry via RT]

In src/lib/gssapi/krb5/s4u_gss_glue.c,
krb5_gss_acquire_cred_impersonate_name doesn't initialize cred.  If
kg_impersonate_name returns an error it doesn't set cred either so when
*output_cred_handle is set to cred it's set to unitialized memory.  The
result is that gss_add_cred_impersonate_name in
src/lib/gssapi/mechglue/g_acquire_cred_imp_name.c will then call
mech->gss_release_cred on a bad cred pointer in its errout section.