decoder: fix data race in input_DecoderIsEmpty()

decoder: fix data race in input_DecoderIsEmpty()

p_dec->fmt_out is owned by the decoder plugin, and can only safely be
accessed by the core from within decoder callbacks, notably the format
update callbacks.

Outside that context, p_owner->fmt has to be used. It contains a copy
of p_dec->fmt_out at the last format update.