Fixed not initialized QVideoFrame mappedCount member

Change-Id: Ic0b915df120222b0bb440698c010f0e83ccce8c3
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimedia/video/qvideoframe.cpp

@@ -82,6 +82,7 @@ public:
         , pixelFormat(QVideoFrame::Format_Invalid)
         , fieldType(QVideoFrame::ProgressiveFrame)
         , buffer(0)
+        , mappedCount(0)
     {
     }