Stay away from unnecessary code paths if the movie is 0 (NULL)

When closing a QT7PlayerSession, it sets the movie to 0. There is no
need to check if a NULL movie supports the core pixel formats, and
that check lead to a segmentation fault with QtSDK 1.1.1.

Task-number: QTMOBILITY-1746
Reviewed-by: Dmytro Poplavskiy
(cherry picked from commit b5d9964aab96ef1a02ca1af411178f05c42c2bf8)

Change-Id: I7cdddbd8fc1b8d8d667747aec3567aeb857accd6
Reviewed-on: http://codereview.qt.nokia.com/984
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Michael Goddard

src/plugins/qt7/qt7movieviewrenderer.mm

@@ -283,6 +283,8 @@ void QT7MovieViewRenderer::setupVideoOutput()
 
         [movieView setMovie:(QTMovie*)m_movie];
         [movieView setDrawRect:QRect(QPoint(0,0), m_nativeSize)];
+    } else {
+        m_nativeSize = QSize();
     }
 
     if (m_surface && !m_nativeSize.isEmpty()) {