Fix a crash in rpc-grind when scanning a non-RPC service

scripts/rpc-grind.nse

@@ -103,6 +103,7 @@ local isRPC = function(host, port)
       -- If we got response, set port to open
       nmap.set_port_state(host, port, "open")
 
+      if #data >= 8 then
         rxid, msgtype = string.unpack(">I4 I4", data)
         -- If response XID does match request XID
         -- and message type equals 1 (REPLY) then
@@ -112,6 +113,7 @@ local isRPC = function(host, port)
         end
       end
     end
+  end
   stdnse.debug1("RPC checking function response data is not RPC.")
 end