Don't use the captured length, and don't fetch the version until we need it.

The captured length reflects the way the capture was done; it should not
affect actual lengths used in the dissection.

Don't fetch the version until we need it; that lets us dissect more of
the packet if the previous change caused us to throw an exception trying
to fetch the version from the correct location rather than not throwing
an exception by fetching it from an incorrect location that happens to
be within the captured data.

Change-Id: I9f63afd4ef51f46c19b3afd2a651a5bb768fecaf
Reviewed-on: https://code.wireshark.org/review/4101
Reviewed-by: Guy Harris <guy@alum.mit.edu>