Check for received packet in rpc-grind.nse.

The script was checking the returned data, but not the status. When
status is false, data is actually an error message. So in case of a
timeout, the script was receiving nil, "TIMEOUT" and interpreting
"TIMEOUT" as a response from the server. It looked like this:

Discovered open port 1434/udp on 127.0.0.1
NSE: rpc-grind: RPC checking function response data is not RPC.
NSE: Target port 1434 is not a RPC port.

This was reported by Christopher Clements.
http://seclists.org/nmap-dev/2012/q3/685

scripts/rpc-grind.nse

@@ -88,8 +88,8 @@ local isRPC = function(host, port)
         end
 
         -- And check response
-        _, data = rpcConn:ReceivePacket()
-        if not data then
+        status, data = rpcConn:ReceivePacket()
+        if not status then
             stdnse.print_debug("%s: isRPC didn't receive response.", SCRIPT_NAME)
             return
         else