Fix krb5 gss_acquire_cred_impersonate_name crash

If gss_acquire_cred_impersonate_name is called using an
impersonator_cred_handle acquired with GSS_C_ACCEPT, we could
dereference null fields of the cred handle and crash.  Fix this by
checking the impersonator_cred_handle usage and returning
GSS_S_NO_CRED if it isn't what we expect, just as we do in
init_sec_context.

Based on a patch from Solly Ross <sross@redhat.com>.

ticket: 8017 (new)
target_version: 1.13
tags: pullup