ffmpeg: only count got_output/errors in decode_error_stat

If threading is used, the first (thread_count - 1) packets are read
before any frame/error is returned. Counting this as successful decoding
is wrong, because it also happens when no single frame could be decoded.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>