Don't use proto_tree_add_expert() with an item rather than a tree.

proto_tree_add_expert() is to be used only on an item that's been
created as a tree (i.e., an interior node, complete with an ett_ value);
to hang expert info off of an *item*, which might *not* have been set up
to take subtree items, use expert_add_info().

Change-Id: Icdcdf39c31c0a03ff80dbb629186e8f6fa82e8ab
Reviewed-on: https://code.wireshark.org/review/3705
Reviewed-by: Guy Harris <guy@alum.mit.edu>