pcapng: respect the fact that wblock->data is a union

Make sure to zero the *entire* thing, and only access it as a given type when
that's the type indicated by the (non-union) type field.

Bug: 10498
Change-Id: I3e94a9c5d399d3ee4aedcd49f1aa2d7678ecf7ce
Reviewed-on: https://code.wireshark.org/review/4273
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>