Fix minor issue introduced with conversion to new expert API in r51688.

Fix minor issue introduced with conversion to new expert API in r51688.
Specifically, proto_tree_add_expert() must take an actual tree node (for example
from proto_item_add_subtree()) and cannot take just any old item node. The
original intent (before the conversion) appeared to be just to put it on the
tree, so do that.

Another assertion gone from
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=9406

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@53456 f5534014-38df-0310-8fa8-9805f1628bb7