EPL: Fix offset calculation

 The offset was calculated too high, as it was added
 to itself and sizes were added multiple times

Change-Id: I1a581e96e2ab66e40f5566074e8bd1089f55bdb0
Reviewed-on: https://code.wireshark.org/review/4049
Reviewed-by: Roland Knall <rknall@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>