avcodec/avcodec: Free frame_thread_encoder on avcodec_open2() error

The frame_thread_encoder has so far not been freed in case an error
happened in avcodec_open2() after ff_frame_thread_encoder_init().
This commit changes this.

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

libavcodec/avcodec.c

@@ -383,6 +383,8 @@ free_and_end:
          avctx->codec->caps_internal & FF_CODEC_CAP_INIT_CLEANUP)))
         avctx->codec->close(avctx);
 
+    if (CONFIG_FRAME_THREAD_ENCODER && avci->frame_thread_encoder)
+        ff_frame_thread_encoder_free(avctx);
     if (HAVE_THREADS && avci->thread_ctx)
         ff_thread_free(avctx);