[cage] Don't deref NULL value in Exception.set_attr_str.
[cage] Don't deref NULL value in Exception.set_attr_str.

This is partial fix for Rakudo's RT#68748 ticket.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@40750 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot