mlx4_en: Fix read buffer overflow in mlx4_en_complete_rx_desc()

If the length is less or equal to frag_prefix_size in the first iteration
we write skb_frags_rx[-1] and read from priv->frag_info[-1]

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>