Fixup the wireshark lua packet dissector heuristic filter

This fixes a bug where the first SCION packet in a capture is never
dissected as a SCION packet.

The filter function is (only) called on the first packet of the
conversation (4-tuple) since we set the dissector for the conversation.

To also dissect the initial packet of a conversation for which a
heuristic dissector function is registered, the dissector has to be
called explicitly at the end of the filter function when a packet matches.

From the docs:
https://www.wireshark.org/docs/wsdg_html/#lua_fn_proto_register_heuristic_listname__func_
>The function should perform as much verification as possible to ensure
the payload is for it, and **dissect the packet** (including setting
TreeItem info and such) only if the payload is for it, before returning
true or false.

Also look at the comments there:
https://github.com/wireshark/wireshark/blob/master/test/lua/dissector.lua#L523
https://github.com/wireshark/wireshark/blob/afff4e02/test/lua/dissector.lua#L523

tools/wireshark/scion.lua

@@ -143,11 +143,17 @@ local function scion_proto_filter(tvbuf, pktinfo, root)
     local addr_type_src_valid = (addrTypes[bit.band(tvbuf(9, 1):uint(), 0xf)] ~= nil)
     local rsv_valid = (tvbuf(10, 2):uint() == 0)
 
-    if version_valid and next_hdr_valid and path_type_valid and addr_type_dst_valid and addr_type_src_valid and rsv_valid then
-       pktinfo.conversation = scion_proto
-       return true
+    if not (version_valid and next_hdr_valid and path_type_valid and addr_type_dst_valid and addr_type_src_valid and rsv_valid) then
+       return false
     end
-    return false
+
+    -- This looks like a SCION packet, dissect it.
+    scion_proto.dissector(tvbuf, pktinfo, root)
+    -- Set the scion_proto dissector for the conversation, so that all packets with the same
+    -- 4-tuple use directly the scion_proto dissector (instead of calling again this
+    -- heuristic function).
+    pktinfo.conversation = scion_proto
+    return true
 end
 
 function scion_proto.dissector(tvbuf, pktinfo, root)