pgsql: Fix unnest() to handle a toasted input array safely.

Log Message:
-----------
Fix unnest() to handle a toasted input array safely. Per report from
Alvaro.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
arrayfuncs.c (r1.152 -> r1.153)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/arrayfuncs.c?r1=1.152&r2=1.153)