dtc: Fix NULL pointer use in dtlabel + dtref case

If we have a construct like this:

	label: &handle {
		...
	};

Running dtc on it will cause a segfault, because we use 'target'
when it could be NULL. Move the add_label() call into the if
statement to fix this potentially bad use of a NULL pointer.

Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc-parser.y

@@ -171,10 +171,10 @@ devicetree:
 		{
 			struct node *target = get_node_by_ref($1, $3);
 
-			add_label(&target->labels, $2);
-			if (target)
+			if (target) {
+				add_label(&target->labels, $2);
 				merge_nodes(target, $4);
-			else
+			} else
 				ERROR(&@3, "Label or path %s not found", $3);
 			$$ = $1;
 		}