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 17aeee58)

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;
 }