make sure opencv builds fine with the recent versions of libav (e.g. from Ubuntu 14.10)

modules/videoio/test/test_ffmpeg.cpp

@@ -132,6 +132,7 @@ public:
                         writer << img;
                     }
 
+                    writer.release();
                     if (!created) created = true;
                     else remove(filename.c_str());
                 }