os-util: use _IMAGE_CLASS_INVALID

When extension is not specified, image class is not necessary to be
specified. Let's use _IMAGE_CLASS_INVALID as an indicator that no
extension is specified.

src/basic/os-util.c

@@ -65,8 +65,6 @@ int path_is_extension_tree(ImageClass image_class, const char *path, const char
         int r;
 
         assert(path);
-        assert(image_class >= 0);
-        assert(image_class < _IMAGE_CLASS_MAX);
 
         /* Does the path exist at all? If not, generate an error immediately. This is useful so that a missing root dir
          * always results in -ENOENT, and we can properly distinguish the case where the whole root doesn't exist from
@@ -305,7 +303,7 @@ int _parse_os_release(const char *root, ...) {
         int r;
 
         va_start(ap, root);
-        r = parse_release_internal(root, -1, /* relax_extension_release_check= */ false, NULL, ap);
+        r = parse_release_internal(root, _IMAGE_CLASS_INVALID, /* relax_extension_release_check= */ false, NULL, ap);
         va_end(ap);
 
         return r;