checks: Fix segmentation fault in check_graph_node

Dts files which contain an 'endpoint' node as a direct child of the
root node cause a segmentation fault inside check_graph_node(). This
type of error can easily happen when a 'remote-endpoint' property is
accidentally placed outside the corresponding endpoint and port nodes.

Example with 'endpoint' node:
/dts-v1/;
/ {	endpoint {};  };

Example with remote-endpoint property:
/dts-v1/;
/ {
	foo {
                remote-endpoint = <0xdeadbeef>;
	};
};

Signed-off-by: Johannes Beisswenger <johannes.beisswenger@cetitec.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

checks.c

@@ -1767,6 +1767,11 @@ static void check_graph_nodes(struct check *c, struct dt_info *dti,
 		      get_property(child, "remote-endpoint")))
 			continue;
 
+                /* The root node cannot be a port */
+		if (!node->parent) {
+			FAIL(c, dti, node, "root node contains endpoint node '%s', potentially misplaced remote-endpoint property", child->name);
+			continue;
+		}
 		node->bus = &graph_port_bus;
 
 		/* The parent of 'port' nodes can be either 'ports' or a device */