Fix a small memory leak.
Fix a small memory leak.

- Malloced pointer returned by alloc_yuv_t was not being freed in
  substream_encode.

- Remove use of yuv_t from encode_one_frame, as it's not used there anymore.

lu-zero/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,lu-zero/kvazaar,lu-zero/kvazaar,lu-zero/kvazaar,ultravideo/kvazaar