Fix memory leaks in chew program.

	* doc/chew.c (delete_string): Only free the string buffer if it is
	there.  Mark the buffer as NULL after freeing.
	(drop): Free the dropped string.
	(free_words): New function: Frees the memory allocated to the
	dictionary.
	(add_instrinsic): Duplicate the name string, so that it can be
	freed later on.
	(compile): Free unused words.
	(main): Free the dictionary and top level string buffers at the
	end.