decoder_ffmpeg: replace the assert in ffdec_uninit_hw with the corresponding check
decoder_ffmpeg: replace the assert in ffdec_uninit_hw with the corresponding check

It could happen that a software decoder is already associated with the
AVCodecContext passed to ffdec_init_hw. If this function fails,
ffdec_uninit_hw is called triggering the assert (which is something that
we do not want).

Stupeflix/sxplayer,Stupeflix/sxplayer,Stupeflix/sxplayer