ffmpeg_vaapi: Unreference global device before setting it

This currently leaks if multiple device options are specified on the
command line.

ffmpeg_vaapi.c

@@ -220,6 +220,8 @@ av_cold int vaapi_device_init(const char *device)
 {
     int err;
 
+    av_buffer_unref(&hw_device_ctx);
+
     err = av_hwdevice_ctx_create(&hw_device_ctx, AV_HWDEVICE_TYPE_VAAPI,
                                  device, NULL, 0);
     if (err < 0) {