Fix crasher bugs in previous commit

ALTER DEFAULT PRIVILEGES was trying to decode the list of roles in the
FOR clause as a list of names rather than of RoleSpecs; and the IN
clause in CREATE ROLE was doing the same thing.  This was evidenced by
crashes on some buildfarm machines, though on my platform this doesn't
cause a failure by mere chance; I can reproduce the failures only by
adding some padding in struct RoleSpecs.

Fix by dereferencing those lists as being of RoleSpecs, not string
Values.