heimdal: Fix uninitialized pointer dereference

krb5_ret_preincipal() returns a non-zero return code when
a garbage principal is passed to it. Unfortunately ret_principal_ent()
does not check the return code, with garbage pointing to what would
have been the principal. This results in a segfault when free() is
called.

PR: 267944, 267972
Reported by: Robert Morris <rtm@lcs.mit.edu>
MFC after: 3 days