Fix plpgsql's exec_eval_expr() to ensure it returns a sane type OID

even when the expression is a query that returns no rows.

So far as I can tell, the only caller that actually fails when a garbage
OID is returned is exec_stmt_case(), which is new in 8.4 --- in all other
cases, we might make a useless trip through casting logic, but we won't
fail since the isnull flag will be set.  Hence, backpatch only to 8.4,
just in case there are apps out there that aren't expecting an error to
be thrown if the query returns more or less than one column.  (Which seems
unlikely, since the error would be thrown if the query ever did return a
row; but it's possible there's some never-exercised code out there.)

Per report from Mario Splivalo.