bnx2x: fix possible panic under memory stress

While it is legal to kfree(NULL), it is not wise to use :
put_page(virt_to_head_page(NULL))

 BUG: unable to handle kernel paging request at ffffeba400000000
 IP: [<ffffffffc01f5928>] virt_to_head_page+0x36/0x44 [bnx2x]
Reported-by: NMichel Lespinasse <walken@google.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Ariel Elior <ariel.elior@qlogic.com>
Fixes: d46d132c ("bnx2x: use netdev_alloc_frag()")
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bnx2x/bnx2x_cmn.c

@@ -797,7 +797,8 @@ static void bnx2x_tpa_stop(struct bnx2x *bp, struct bnx2x_fastpath *fp,
 
 		return;
 	}
-	bnx2x_frag_free(fp, new_data);
+	if (new_data)
+		bnx2x_frag_free(fp, new_data);
 drop:
 	/* drop the packet and keep the buffer in the bin */
 	DP(NETIF_MSG_RX_STATUS,