PG 11 JIT deform failure

Hi,

JIT slot_compile_deform assumes there's at least  'natts' in TupleDesc, eg
        /*
         * Iterate over each attribute that needs to be deformed, build code to
         * deform it.
         */
        for (attnum = 0; attnum < natts; attnum++)
        {
                Form_pg_attribute att = TupleDescAttr(desc, attnum);

but a new TupleDesc has no attribute and the caller only tests
TupleDesc is not null.