copying portion of video produces broken audio and video

[matteosistisette]

Steps to reproduce:
- open a video
- select an in and out point (A and B)
- select Video: "copy" and Audio: "copy" (which are the default values).
- save the video

Result:
- the audio is accelerated and noisy (full of clicks)
- the first few video frames are almost green
- in and out point are not accurate: e.g. you have selected from frame M to frame N, and it copies from M+x to N+y where x and y are some arbitrary random positive or negative integers.