tools: fix wireshark dissector TLV options without data (scionproto#4458)

Dissecting TLV options crashes the wireshark plugin, if the `data` field
is empty. This happens always for Pad1 options and for PadN options if
`data_len` is 0.

Author: marcodermatt

tools/wireshark/scion.lua

@@ -558,7 +558,9 @@ function scion_extn_tlv_option_dissect(tvbuf, pktinfo, root)
     else
         -- no specific dissector
         ret_len = data_len
-        tree:add(scion_extn_tlv_option_value, tlv.data)
+        if tlv.data ~= nil then
+            tree:add(scion_extn_tlv_option_value, tlv.data)
+        end
     end
 
     local type_str = scion_extn_tlv_option_types[tlv.type:uint()]