wslua: fail on tree:add(nil)

It is unlikely that tree:add(nil) or tree:add(nil, tvb) represents a
valid invocation. Most likely the developer made a mistake and tried
something like:

    local pf_thing = ProtoField.string("some.thing", "Description")
    some_proto.fields = { pf_thing }
    ...
        tree:add(some_proto.fields.thing, tvb())

which should have been:

    some_proto.fields = { thing = pf_thing }

or:

        tree:add(pf_thing, tvb())

Save the developer some pain and bail out early.

Change-Id: I59fc04153f02bb186b47b763676a68bb0271df76
Reviewed-on: https://code.wireshark.org/review/16177
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/epan/wslua/wslua_tree.c b/epan/wslua/wslua_tree.c
index 1ca9794a55..716efec384 100644
--- a/epan/wslua/wslua_tree.c
+++ b/epan/wslua/wslua_tree.c
@@ -324,6 +324,8 @@ static int TreeItem_add_item_any(lua_State *L, gboolean little_endian) {
             hfid = proto->hfid;
             type = FT_PROTOCOL;
             ett = proto->ett;
+        } else if (lua_isnil(L, 1)) {
+            return luaL_error(L, "first argument to TreeItem:add is nil!");
         }
     } else {
         hfid = field->hfid;