drivers: base: swnode: check if swnode is NULL before dereferencing it

The to_software_mode() macro can potentially return NULL, so also add
a NULL check on swnode before dereferencing it to avoid any NULL
pointer dereferences.

Detected by CoverityScan, CID#1476052 ("Explicit null dereferenced")

Fixes: 59abd836 (drivers: base: Introducing software nodes to the firmware node framework)
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/swnode.c

@@ -477,7 +477,8 @@ software_node_get_parent(const struct fwnode_handle *fwnode)
 {
 	struct software_node *swnode = to_software_node(fwnode);
 
-	return swnode->parent ? &swnode->parent->fwnode : NULL;
+	return swnode ? (swnode->parent ? &swnode->parent->fwnode : NULL) :
+			NULL;
 }
 
 struct fwnode_handle *