pgsql: plpgsql's exec_simple_cast_value() mistakenly supposed that it

Log Message:
-----------
plpgsql's exec_simple_cast_value() mistakenly supposed that it could bypass
casting effort whenever the input value was NULL. However this prevents
application of not-null domain constraints in the cases that use this
function, as illustrated in bug #4741. Since this function isn't meant
for use in performance-critical paths anyway, this certainly seems like
another case of "premature optimization is the root of all evil".

Back-patch as far as 8.2; older versions made no effort to enforce
domain constraints here anyway.

Tags:
----
REL8_2_STABLE

Modified Files:
--------------
pgsql/src/pl/plpgsql/src:
pl_exec.c (r1.180.2.9 -> r1.180.2.10)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c?r1=1.180.2.9&r2=1.180.2.10)