[netdevice] Do not attempt to unmap a null I/O buffer

netdev_tx_err() may be called with a null I/O buffer (e.g. to record a
transmit error with no associated buffer).  Avoid a potential null
pointer dereference in the DMA unmapping code path.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdevice.c

@@ -392,7 +392,7 @@ void netdev_tx_err ( struct net_device *netdev,
 	}
 
 	/* Unmap I/O buffer, if required */
-	if ( dma_mapped ( &iobuf->map ) )
+	if ( iobuf && dma_mapped ( &iobuf->map ) )
 		iob_unmap ( iobuf );
 
 	/* Discard packet */