Fixed #12151: Ensured the comments code does not cause a server error when a...

Fixed #12151: Ensured the comments code does not cause a server error when a request comes in for a comment specifying an invalid primary key value. Thanks thejaswi_puthraya.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12681 bcc190cf-cafb-0310-a4f2-bffc1f526a37