net: avoid infinite loop when receiving packets(CVE-2015-5278)

Ne2000 NIC uses ring buffer of NE2000_MEM_SIZE(49152)
bytes to process network packets. While receiving packets
via ne2000_receive() routine, a local 'index' variable
could exceed the ring buffer size, leading to an infinite
loop situation.

Reported-by: Qinghao Tang <luodalongde@gmail.com>
Signed-off-by: P J P <pjp@fedoraproject.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/net/ne2000.c

diff --git a/hw/net/ne2000.c b/hw/net/ne2000.c
index 3798a3b..010f9ef 100644
--- a/hw/net/ne2000.c
+++ b/hw/net/ne2000.c
@@ -247,7 +247,7 @@
         if (index <= s->stop)
             avail = s->stop - index;
         else
-            avail = 0;
+            break;
         len = size;
         if (len > avail)
             len = avail;