dissect: do not trigger assertion on error

recurse_dir() does not provide 'sx' on error.

Fixes #25862.

src/dissect/dissect.c

@@ -901,11 +901,12 @@ static int mtree_print_item(
         int r;
 
         assert(path);
-        assert(sx);
 
         if (IN_SET(event, RECURSE_DIR_ENTER, RECURSE_DIR_ENTRY)) {
                 _cleanup_free_ char *escaped = NULL;
 
+                assert(sx);
+
                 if (isempty(path))
                         path = ".";
                 else {