[1.1.X] Fixed #12151: Ensured the comments code does not cause a server error...

[1.1.X] 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. 

r12681 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12682 bcc190cf-cafb-0310-a4f2-bffc1f526a37