pgsql: Fix failure to apply domain constraints to a NULL constant that's

Log Message:
-----------
Fix failure to apply domain constraints to a NULL constant that's added to
an INSERT target list during rule rewriting. Per report from John Supplee.

Modified Files:
--------------
pgsql/src/backend/rewrite:
rewriteManip.c (r1.94 -> r1.95)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/rewrite/rewriteManip.c.diff?r1=1.94&r2=1.95)