PeTrack crashes when loading project with retrack enabled

When loading a project with enabled retrack PeTrack crashes, due to the following code. When frame and prevFrame are equal, dir becomes 0 which leads to a division by 0, leading to a crash.

Example file: `experiments/2021.06.30_Juelich_IEK6H3_EngstelleMultiplePersons/export/cameras/Sequences/cam0/080_l020_h1_p0_a0012_cam0.pet

bool applyReTrack = reTrack;
if(reTrack)
{
    int dir = (frame - prevFrame); // direction of tracking - forward/backwards
    dir /= std::abs(dir); // theoretically possible to omit MAX_STEP_TRACK frames
    constexpr int minQualReco = 90;
    for(int j = 0;
        at(i).trackPointExist(frame + j*dir) && at(i).trackPointAt(frame + j*dir).qual() < minQualReco;
        ++j){
        if(at(i).trackPointAt(frame + j*dir).qual() < reQual){
            applyReTrack = false;
            break;
        }
    }
}