Fixed post deletion

When post was deleted foreign keys in corresponding comments weren't NULLified.

src/Models/Model_Comment_QueryBuilder.php

@@ -3,7 +3,11 @@ class Model_Comment_QueryBuilder implements AbstractQueryBuilder
 {
 	public static function build($dbQuery, $query)
 	{
-		$dbQuery->from('comment');
-		$dbQuery->orderBy('id')->desc();
+		$dbQuery
+			->from('comment')
+			->where('post_id')
+			->is()->not('NULL')
+			->orderBy('id')
+			->desc();
 	}
 }