access: rist: fix potential endless loop

The loop compared processed_bytes (uint16) with len (size_t), so if len happens to be larger than UINT16_MAX, this condition would be always true.

The value of len is capped by the "packet-size" option, which defaults to RIST_MAX_PACKET_SIZE, so for this case to occur it would require the user changing the option.

Found with lgtm.com