Fix resource leak in directshow plugin.

Add two missing release calls to free file handles.

Task-number: QTBUG-23822
Change-Id: I8f5b6d0dcf9ad66bdaa2f378c0d3e401388d7add
Reviewed-by: Wouter Huysentruit <wouter.huysentruit@dzine.be>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>