wslua: fix error handling for invalid dissector table

Calling DissectorTables's try method for a dissector table of an unknown
type crashes Wireshark.

local dt = DissectorTable.get("iso14443.subdissector")
dt:try(0, tvbuf, pinfo, tree)

causes a segmentation fault

Thread 1 "wireshark" received signal SIGSEGV, Segmentation fault.
except_pop () at /media/sf_wireshark.git/epan/except.c:264
264       set_top(top->except_down);
(gdb) print top
$1 = (struct except_stacknode *) 0x2
(gdb) bt
    at /media/sf_wireshark.git/epan/packet.c:590

My gut feeling (I haven't verified this) is that we should not call luaL_error()
inside a TRY-CATCH block. DissectorTable_try does this when the type of the
dissector table is not supported.

Fall back to the data dissector in this case and bring up an expert info
instead of aborting the dissection completely.

Change-Id: I9a49f738a99b2618014f41050d8c0bf6bfbb4138
Reviewed-on: https://code.wireshark.org/review/33357
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

epan/wslua/wslua_dissector.c

@@ -541,7 +541,7 @@ WSLUA_METHOD DissectorTable_try (lua_State *L) {
                 handled = TRUE;
             }
         } else {
-            luaL_error(L,"No such type of dissector_table");
+            error = "No such type of dissector table";
         }
 
         if (!handled) {