xnu: Add missing error check.

Found by: Coveriy scan.

grub-core/loader/xnu.c

@@ -1378,6 +1378,8 @@ grub_xnu_fill_devicetree (void)
     name[len] = 0;
 
     curvalue = grub_xnu_create_value (curkey, name);
+    if (!curvalue)
+      return grub_errno;
     grub_free (name);
    
     data = grub_malloc (grub_strlen (var->value) + 1);