don't skip another 6 bytes after OPCR
Bug: 10446

simplify OPCR calculation while we're at it

Change-Id: I8590e409895e712fe6bbb64ab23093caf5795fa7
Reviewed-on: https://code.wireshark.org/review/4040
Petri-Dish: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
(cherry picked from commit 4af01a8f30722e969046ab3cd7a1e3e3e4e3d69e)
Reviewed-on: https://code.wireshark.org/review/4041