handle EOF correctly in parser and lexer

* module/language/elisp/lexer.scm (lex, get-lexer/1): Return a valid
  token at EOF.
* module/language/elisp/parser.scm (get-expression): Raise an error if
  EOF is reached.
  (read-elisp): If at EOF, return the EOF object instead of attempting
  to read an expression.

module/language/elisp/parser.scm

@@ -178,6 +178,8 @@
                         (source-properties token)))
                    result)))
     (case type
+      ((eof)
+       (parse-error token "end of file during parsing"))
       ((integer float symbol character string)
        (return (cdr token)))
       ((quote backquote unquote unquote-splicing)
@@ -207,6 +209,9 @@
   (with-fluids ((circular-definitions (make-circular-definitions)))
     (let* ((lexer (get-lexer port))
            (lexbuf (make-lexer-buffer lexer))
-           (result (get-expression lexbuf)))
-      (lexbuf 'finish)
-      result)))
+           (next (lexbuf 'peek)))
+      (if (eq? (car next) 'eof)
+          (cdr next)
+          (let ((result (get-expression lexbuf)))
+            (lexbuf 'finish)
+            result)))))