In dns-zone-transfer.nse, return "<parse error>" in case domain decoding fails.

otherwise there is an assertion failure trying to insert a nil into an output
table in tab.lua. Increase the maximum number of message compression pointers
we will follow in dns.lua from 3 to 10, because Brandon found a server that
used 4..

nselib/dns.lua

@@ -486,7 +486,7 @@ function decStr(data, pos)
       local part
 
       -- Avoid infinite recursion on malformed compressed messages.
-      limit = limit or 3
+      limit = limit or 10
       if limit < 0 then
          return pos, nil
       end