letting my editor remove trailing whitespace. this way my next commit will be atomic
general good-coding principle: give variables the smallest scope necessary
without seeing (newly-added) assertions like these, i might worry that the author of this function forgot to notice that char buffers product_id_return and model_id_return are never visibly zero-initialized in this function, and therefore give the appearance that they may on occasion contain garbage that we then read from.
i won't bother to duplicate this assert *everywhere*. however, it seems appropriate to make sure it asserts for at least 1 call-site per function, for these functions: send_xid_command_pst_proof, send_xid_command, and send_xid_command_slow