Fix for PHP Notice being generated.

"PHP Notice:  Trying to get property of non-object in /comment/main.php on line 325"
Details: We don't need to try and get comments for images that don't exist. This might also help speed things up a bit as it should eliminate an unnecessary database query.
Merge branch 'master' of git://github.com/shish/shimmie2
This should have been moved earlier in the fix for Issue 346.
Merge pull request #358 from jgen/master

PHP Notice being generated in comment.php