Fix resource leak in directshow plugin.

Add two missing release calls to free file handles.

Task-number: QTBUG-23822
Change-Id: I27b631e28d48b7a20cc76c0913a8d962ee991ce5
Reviewed-by: Wouter Huysentruit <wouter.huysentruit@dzine.be>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>