i2c: smbus: Check for parent device before dereference

An I²C adapter might be instantiated without parent. In such case
there is no property can be retrieved. Skip SMBus alert setup when
this happens.

Fixes: a263a84 ("i2c: smbus: Use device_*() functions instead of of_*()")
Reported-by: syzbot+0591ccf54ee05344e4eb@syzkaller.appspotmail.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/i2c-core-smbus.c

@@ -705,10 +705,14 @@ EXPORT_SYMBOL_GPL(i2c_new_smbus_alert_device);
 #if IS_ENABLED(CONFIG_I2C_SMBUS)
 int i2c_setup_smbus_alert(struct i2c_adapter *adapter)
 {
+	struct device *parent = adapter->dev.parent;
 	int irq;
 
-	irq = device_property_match_string(adapter->dev.parent, "interrupt-names",
-					   "smbus_alert");
+	/* Adapter instantiated without parent, skip the SMBus alert setup */
+	if (!parent)
+		return 0;
+
+	irq = device_property_match_string(parent, "interrupt-names", "smbus_alert");
 	if (irq == -EINVAL || irq == -ENODATA)
 		return 0;
 	else if (irq < 0)