columns.  The returned tuple needs to have appropriate NULL columns
inserted so that it actually matches the declared rowtype.  It seemed
convenient to use a JunkFilter for this, so I made some cleanups and
simplifications in the JunkFilter code to allow it to support this
additional functionality.  (That in turn exposed a latent bug in
nodeAppend.c, which is that it was returning a tuple slot whose
descriptor didn't match its data.)  Also, move check_sql_fn_retval
out of pg_proc.c and into functions.c, where it seems to more naturally
belong.