pgsql: Make array_cat more paranoid about checking datatypes in empty

Log Message:
-----------
Make array_cat more paranoid about checking datatypes in empty arrays.

Tags:
----
REL7_4_STABLE

Modified Files:
--------------
pgsql/src/backend/utils/adt:
array_userfuncs.c (r1.11 -> r1.11.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/array_userfuncs.c.diff?r1=1.11&r2=1.11.2.1)