Fix a memory leak in CvCapture_FFMPEG::close

FFmpeg now requires that frames allocated with avcodec_alloc_frame are
freed with avcodec_free_frame.

(cherry picked from commit 77578d41)

modules/highgui/src/cap_ffmpeg_impl.hpp

@@ -292,7 +292,15 @@ void CvCapture_FFMPEG::close()
     }
 
     if( picture )
+    {
+        // FFmpeg and Libav added avcodec_free_frame in different versions.
+#if LIBAVCODEC_BUILD >= (LIBAVCODEC_VERSION_MICRO >= 100 \
+    ? CALC_FFMPEG_VERSION(54, 59, 100) : CALC_FFMPEG_VERSION(54, 28, 0))
+        avcodec_free_frame(&picture);
+#else
         av_free(picture);
+#endif
+    }
 
     if( video_st )
     {