then ff_mpv_encode_end() will be unnecessary in ff_mpv_encode_init()
if it's failed.

The FF_CODEC_CAP_INIT_CLEANUP flag is need for single thread, For multithread,
it'll be cleanup still by AV_CODEC_CAP_FRAME_THREADS flag if have.
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>