iio: adc: stm32-adc: harden against NULL pointer deref in stm32_adc_probe()

of_match_device() may fail and returns a NULL pointer.

In practice there is no known reasonable way to trigger this, but
in case one is added in future, harden the code by adding the check
Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Link: https://lore.kernel.org/r/tencent_994DA85912C937E3B5405BA960B31ED90A08@qq.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc-core.c

@@ -708,6 +708,8 @@ static int stm32_adc_probe(struct platform_device *pdev)
 	struct stm32_adc_priv *priv;
 	struct device *dev = &pdev->dev;
 	struct device_node *np = pdev->dev.of_node;
+	const struct of_device_id *of_id;
+
 	struct resource *res;
 	u32 max_rate;
 	int ret;
@@ -720,8 +722,11 @@ static int stm32_adc_probe(struct platform_device *pdev)
 		return -ENOMEM;
 	platform_set_drvdata(pdev, &priv->common);
 
-	priv->cfg = (const struct stm32_adc_priv_cfg *)
-		of_match_device(dev->driver->of_match_table, dev)->data;
+	of_id = of_match_device(dev->driver->of_match_table, dev);
+	if (!of_id)
+		return -ENODEV;
+
+	priv->cfg = (const struct stm32_adc_priv_cfg *)of_id->data;
 	priv->nb_adc_max = priv->cfg->num_adcs;
 	spin_lock_init(&priv->common.lock);