fix for having too many classes

[trobro]

After initializing with a rect close to the image edge I sometimes got a crash in CMT::processFrame() due to classes_active.size() being greater than the size of the status vector. This patch makes sure that classes_active isn't bigger than the number of stored keypoints, which could previously happen when keypoints were removed by descriptor->compute() in the initialization.

CppMT is great btw, thanks for creating the project!