fdt: Check for a valid fdt in oftree_ensure()

Check the header before starting to use it, since this could provide
very confusing later, when ofnode calls start to fail.

Signed-off-by: Simon Glass <sjg@chromium.org>

drivers/core/ofnode.c

@@ -83,6 +83,11 @@ static oftree oftree_ensure(void *fdt)
 			if (check_tree_count())
 				return oftree_null();
 
+			if (fdt_check_header(fdt)) {
+				log_err("Invalid device tree blob header\n");
+				return oftree_null();
+			}
+
 			/* register the new tree */
 			i = oftree_count++;
 			oftree_list[i] = fdt;