Because nothing guarantees or enforces that either Vtreesit_thing_settings,
or the cdrs of its elements, are cons cells, it is easy to construct values
for `treesit-thing-settings' that make Emacs crash (e.g., when calling
`prog-fill-reindent-defun' from any tree-sitter enabled mode).

The attached patch fixes that.

--
 Dominique Quatravaux
 dominique@quatravaux.org