Always create subtree in proto_item_add_subtree().

It might fix bug #10345

Ping-Bug: 10345
Change-Id: I695a2955a9bb0248ff3967bdaef004389500bbc9
Reviewed-on: https://code.wireshark.org/review/3402
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Tested-by: Anders Broman <a.broman58@gmail.com>
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>