avcodec/utils: Improve check for freeing codec private options

Don't check for AVCodec.priv_data_size (which is always true if
AVCodec.priv_class is set). Instead check for AVCodecContext.priv_data
to actually exist.

(Note: av_opt_free(NULL) is a no-op.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/utils.c

@@ -1030,7 +1030,7 @@ free_and_end:
     if (HAVE_THREADS && avci->thread_ctx)
         ff_thread_free(avctx);
 
-    if (codec->priv_class && codec->priv_data_size)
+    if (codec->priv_class && avctx->priv_data)
         av_opt_free(avctx->priv_data);
     av_opt_free(avctx);