pgsql: Fix crasher bugs in previous commit

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.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/e3f1c24b992acb88e4ccf33118640aee4b11dd47

Modified Files
--------------
src/backend/catalog/aclchk.c |   20 ++++++++++----------
src/backend/commands/user.c  |    8 ++++++--
2 files changed, 16 insertions(+), 12 deletions(-)