Add a couple more assertions to the module code

Add a couple more assertions to the module code

These can help module authors debug crashes.

* emacs-module.c (module_non_local_exit_check)
(module_non_local_exit_clear, module_non_local_exit_get)
(module_non_local_exit_signal, module_non_local_exit_throw)
(module_copy_string_contents, module_make_string)
(funcall_module, initialize_environment): Add assertions