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

The pointer p can be potentially NULL as macro to_software_node can
return NULL.

Add null check on p before dereferencing it to avoid any NULL pointer
dereferences.

Detected by CoverityScan, CID#1476039 ("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

@@ -487,7 +487,7 @@ software_node_get_next_child(const struct fwnode_handle *fwnode,
 	struct software_node *p = to_software_node(fwnode);
 	struct software_node *c = to_software_node(child);
 
-	if (list_empty(&p->children) ||
+	if (!p || list_empty(&p->children) ||
 	    (c && list_is_last(&c->entry, &p->children)))
 		return NULL;