pgsql: Refactor exec_cast_value() and exec_simple_cast_value(): since

Log Message:
-----------
Refactor exec_cast_value() and exec_simple_cast_value(): since they do
not ever write through the `isnull' parameter, it does not need to be
an out parameter. Therefore it can be declared a "bool" rather than a
"bool *".

Modified Files:
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pgsql/src/pl/plpgsql/src:
pl_exec.c (r1.150 -> r1.151)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.150&r2=1.151)