Encoder frame_number may be double-counted if some frames are cached and then flushed.
Take qsv encoder (some frames are cached firsty for asynchronism) as example,
./ffmpeg -loglevel verbose -hwaccel qsv -c:v h264_qsv -i in.mp4 -vframes 100 -c:v h264_qsv out.mp4
frame_number passed to encoder is double-counted and larger than the accurate value.
Libx264 encoding with B frames can also reproduce it.

Signed-off-by: Zhong Li <zhong.li@intel.com>