Make lisp-do-defun do the right thing with trailing comments

* lisp/progmodes/inf-lisp.el (lisp-do-defun): Avoid including
trailing comments in the region (bug#7974).

lisp/progmodes/inf-lisp.el

@@ -345,8 +345,11 @@ The actually processing is done by `do-string' and `do-region'
  which determine whether the code is compiled before evaluation.
 DEFVAR forms reset the variables to the init values."
   (save-excursion
-    (end-of-defun)
-    (skip-chars-backward " \t\n\r\f") ;  Makes allegro happy
+    ;; Find the end of the defun this way to avoid having the region
+    ;; possibly end with a comment (it there'a a comment after the
+    ;; final parenthesis).
+    (beginning-of-defun)
+    (forward-sexp)
     (let ((end (point)) (case-fold-search t))
       (beginning-of-defun)
       (if (looking-at "(defvar")