Doc: Fix faulty snippet in Video Overview page

This change modifies setVideoSurface() in the snippet code
to be a bit more robust.

Task-number: QTBUG-29383

Change-Id: I2c691f19fe8f0912c1287597045da3a5579d2108
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

src/multimedia/doc/snippets/multimedia-snippets/video.cpp

@@ -86,7 +86,11 @@ public:
 
     void setVideoSurface(QAbstractVideoSurface *surface)
     {
+        if (m_surface != surface && m_surface && m_surface->isActive()) {
+            m_surface->stop();
+        }
         m_surface = surface;
+        if (m_surface)
             m_surface->start(m_format);
     }