vaapi_encode: Discard output buffer if picture submission fails

Previously this was leaking, though it actually hit an assert making
sure that the buffer had already been cleared when freeing the picture.

(cherry picked from commit 17aeee5832b9188b570c3d3de4197e4cdc54c634)

libavcodec/vaapi_encode.c

@@ -428,6 +428,8 @@ fail:
 fail_at_end:
     av_freep(&pic->codec_picture_params);
     av_frame_free(&pic->recon_image);
+    av_buffer_unref(&pic->output_buffer_ref);
+    pic->output_buffer = VA_INVALID_ID;
     return err;
 }