af_packet driver needs to check VLIB_BUFFER_NEXT_PRESENT flag is set

when walking  vlib_buffer_t next_buffer chain on transmit.
On buffer allocation:
- next_buffer is not and may contain a stale invalid value that
  should be ignored if not overwritten by a valid value.
- VLIB_BUFFER_NEXT_PRESENT flag is cleared and only set
  if a valid value is written to next_buffer.

Change-Id: I9b0ccdc54f4f7456f8328ce7c4a0d52d0fba8caa
Signed-off-by: Jim Gibson <gibson+fdio@cisco.com>

src/vnet/devices/af_packet/device.c

@@ -125,7 +125,8 @@ af_packet_interface_tx (vlib_main_t * vm,
 		       vlib_buffer_get_current (b0), len);
 	  offset += len;
 	}
-      while ((bi = b0->next_buffer));
+      while ((bi =
+	      (b0->flags & VLIB_BUFFER_NEXT_PRESENT) ? b0->next_buffer : 0));
 
       tph->tp_len = tph->tp_snaplen = offset;
       tph->tp_status = TP_STATUS_SEND_REQUEST;