Qt: Clear the packet list selection before freezing it.

Qt: Clear the packet list selection before freezing it.

Call selectionModel()->clear() before removing our model. This will
hopefully make sure we don't end up with an invalid selection in
selectionChanged().

Clear the selection model before clearing the model while we're here.

Change-Id: I1007eeaa480afa0fb31dafc15a49ca6c247b30c8
Reviewed-on: https://code.wireshark.org/review/16062
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Kenny Root <kenny@the-b.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>