proto_add_subtree_format(): initialize tree_item to 'tree' rather than NULL.

... As would be the case if proto_tree_add_text() + proto_item_add_subtree()
were used.  (This initialization value is only used when TRY_TO_FAKE_THIS_ITEM()
shortcuts us out.)

As reported/discussed on -dev:

https://www.wireshark.org/lists/wireshark-dev/201407/msg00031.html

Change-Id: I4af63e3cf0a70607d58b4641597b2ce7907fbb8b
Reviewed-on: https://code.wireshark.org/review/3271
Reviewed-by: Michael Mann <mmann78@netscape.net>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>
Reviewed-by: Evan Huus <eapache@gmail.com>