net: w5100: check return value after calling platform_get_resource()

It will cause null-ptr-deref if platform_get_resource() returns NULL,
we need check the return value.
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/wiznet/w5100.c

@@ -1047,6 +1047,8 @@ static int w5100_mmio_probe(struct platform_device *pdev)
 		mac_addr = data->mac_addr;
 
 	mem = platform_get_resource(pdev, IORESOURCE_MEM, 0);
+	if (!mem)
+		return -EINVAL;
 	if (resource_size(mem) < W5100_BUS_DIRECT_SIZE)
 		ops = &w5100_mmio_indirect_ops;
 	else