i2c: mux: reg: check return value after calling platform_get_resource()

It will cause null-ptr-deref in resource_size(), if platform_get_resource()
returns NULL, move calling resource_size() after devm_ioremap_resource() that
will check 'res' to avoid null-ptr-deref.
And use devm_platform_get_and_ioremap_resource() to simplify code.

Fixes: b3fdd327 ("i2c: mux: Add register-based mux i2c-mux-reg")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/muxes/i2c-mux-reg.c

@@ -183,13 +183,12 @@ static int i2c_mux_reg_probe(struct platform_device *pdev)
 	if (!mux->data.reg) {
 		dev_info(&pdev->dev,
 			"Register not set, using platform resource\n");
-		res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
-		mux->data.reg_size = resource_size(res);
-		mux->data.reg = devm_ioremap_resource(&pdev->dev, res);
+		mux->data.reg = devm_platform_get_and_ioremap_resource(pdev, 0, &res);
 		if (IS_ERR(mux->data.reg)) {
 			ret = PTR_ERR(mux->data.reg);
 			goto err_put_parent;
 		}
+		mux->data.reg_size = resource_size(res);
 	}
 
 	if (mux->data.reg_size != 4 && mux->data.reg_size != 2 &&