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

scripts/dns-zone-transfer.nse

@@ -123,7 +123,9 @@ end
 --@param data String of data.
 --@param offset Offset in the string to read the domain name.
 function parse_domain(data, offset)
-	return dns.decStr(data, offset)
+	local offset, domain = dns.decStr(data, offset)
+	domain = domain or "<parse error>"
+	return offset, domain
 end 
 
 --- Build RFC 1035 root domain name from the name of the DNS server