Fix from Olof Johansson to avoid overwriting valid tx ring entries.

diff -urN linux-2.5/drivers/net/e1000/e1000_main.c ameslab-2.5/drivers/net/e1000/e1000_main.c
--- linux-2.5/drivers/net/e1000/e1000_main.c	2004-03-09 14:32:14.000000000 +1100
+++ ameslab-2.5/drivers/net/e1000/e1000_main.c	2004-02-27 00:45:41.000000000 +1100
@@ -1582,7 +1582,7 @@
 
 	i = tx_ring->next_to_use;
 
-	while(len) {
+	while(len && count < E1000_DESC_UNUSED(&adapter->tx_ring)) {
 		buffer_info = &tx_ring->buffer_info[i];
 		size = min(len, max_per_txd);
 #ifdef NETIF_F_TSO
@@ -1619,7 +1619,7 @@
 		len = frag->size;
 		offset = frag->page_offset;
 
-		while(len) {
+		while(len && count < E1000_DESC_UNUSED(&adapter->tx_ring)) {
 			buffer_info = &tx_ring->buffer_info[i];
 			size = min(len, max_per_txd);
 #ifdef NETIF_F_TSO