pgsql: Fix a couple of places where the plpgsql grammar would produce an

Log Message:
-----------
Fix a couple of places where the plpgsql grammar would produce an unhelpful
'syntax error' message, rather than something that might draw one's
attention to a missing or wrong-type variable declaration.  Per recent
gripe.

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.114 -> r1.115)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.114&r2=1.115)