fftools/ffmpeg: fix the wrong bitrate and speed stats

fftools/ffmpeg: fix the wrong bitrate and speed stats

The bitrate and speed stats are wrong if the copyts is enabled.
Please test with below command:
wget http://samples.mplayerhq.hu/MPEG2/foxksaz.ts
./ffmpeg -y -copyts -i ./foxksaz.ts -c:v libx264 -x264opts \
    nal-hrd=cbr:force-cfr=1 -b:v 3500k -minrate 3500k -maxrate 3500k -bufsize \
    1000k  -c:a mp2 -muxrate 4500k  -vframes 1000 test.ts

before:
frame= 1000 fps=112 q=-1.0 Lsize=    9063kB time=09:10:12.41 bitrate=   2.2kbits/s speed=3.7e+03x
after:
frame= 1000 fps=112 q=-1.0 Lsize=    9062kB time=00:00:15.78 bitrate=4703.4kbits/s speed=1.77x

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>