stats_tree: Make sure some structs are fully initialized.

Get rid of a strcpy.

Change-Id: I49460969fdf627fdb89030115ad5a658e300a5d4
Reviewed-on: https://code.wireshark.org/review/6130
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>