ssb: Fix potential NULL pointer dereference in ssb_device_uevent()

The ssb_device_uevent() function first attempts to convert the 'dev' pointer
to 'struct ssb_device *'. However, it mistakenly dereferences 'dev' before
performing the NULL check, potentially leading to a NULL pointer
dereference if 'dev' is NULL.

To fix this issue, move the NULL check before dereferencing the 'dev' pointer,
ensuring that the pointer is valid before attempting to use it.

Found by Linux Verification Center (linuxtesting.org) with SVACE.
Signed-off-by: Rand Deeb <rand.sec96@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240306123028.164155-1-rand.sec96@gmail.com

drivers/ssb/main.c

@@ -341,11 +341,13 @@ static int ssb_bus_match(struct device *dev, struct device_driver *drv)
 
 static int ssb_device_uevent(const struct device *dev, struct kobj_uevent_env *env)
 {
-	const struct ssb_device *ssb_dev = dev_to_ssb_dev(dev);
+	const struct ssb_device *ssb_dev;
 
 	if (!dev)
 		return -ENODEV;
 
+	ssb_dev = dev_to_ssb_dev(dev);
+
 	return add_uevent_var(env,
 			     "MODALIAS=ssb:v%04Xid%04Xrev%02X",
 			     ssb_dev->id.vendor, ssb_dev->id.coreid,