pgsql: Fix check_sql_fn_retval to allow the case where a SQL function

Log Message:
-----------
Fix check_sql_fn_retval to allow the case where a SQL function declared to
return void ends with a SELECT, if that SELECT has a single result that is
also of type void.  Without this, it's hard to write a void function that
calls another void function.  Per gripe from Peter.

Back-patch as far as 8.0.

Modified Files:
--------------
    pgsql/src/backend/executor:
        functions.c (r1.113 -> r1.114)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/functions.c.diff?r1=1.113&r2=1.114)