[krbdev.mit.edu #7162] krb5_verify_init_creds frees its input argument

[Russ Allbery <rra@stanford.edu> via RT]

Commit caf1fdd98690019d9ac9f56125f4916cfbdfd2d4 introduced a bug in
krb5_verify_init_creds that causes it to unconditionally free the
caller-provided server krb5_principal argument.

The previous code contained this branch to preserve the input argument:

-    if (server_arg) {
-        ret = krb5_copy_principal(context, server_arg, &server);
-        if (ret)
-            goto cleanup;

so that server could later be unconditionally freed.  This
krb5_copy_principal was dropped by the refactoring.  This results in a
double-free for callers, like pam-krb5, that expect to be responsible
for freeing the krb5_principal that they're passing in.