If the NFS/TCP stream is out of sync between the client and server,

and if the client (erroneously) reads the RPC length as 0 bytes, the
client can loop around in the socket callback. Explicitly check for
the length being 0 case and teardown/re-connect.

Submitted by:	Mohan Srinivasan

sys/nfsclient/nfs_socket.c

@@ -836,7 +836,7 @@ nfs_clnt_tcp_soupcall(struct socket *so, void *arg, int waitflag)
 			 * This is SERIOUS! We are out of sync with the sender
 			 * and forcing a disconnect/reconnect is all I can do.
 			 */
-			if (len > NFS_MAXPACKET) {
+			if (len > NFS_MAXPACKET || len == 0) {
 				log(LOG_ERR, "%s (%d) from nfs server %s\n",
 				    "impossible packet length",
 				    len,