lavd/fbdev: pass proper memory map length

fbdev->data was allocated with size fbdev->fixinfo.smem_len, release data
in fbdev_read_close() using the same size.
Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

libavdevice/fbdev.c

@@ -231,7 +231,7 @@ static av_cold int fbdev_read_close(AVFormatContext *avctx)
 {
     FBDevContext *fbdev = avctx->priv_data;
 
-    munmap(fbdev->data, fbdev->frame_size);
+    munmap(fbdev->data, fbdev->fixinfo.smem_len);
     close(fbdev->fd);
 
     return 0;