Incorrect checks in Blob::offset

Code

• branch: master

Description

See the code https://github.com/BVLC/caffe/blob/864520713a4c5ffae7382ced5d34e4cadc608473/include/caffe/blob.hpp#L153-L164

• n should be in the range [0, num())
• c is in [0, channels())
• h is in [0, height())
• w is in [0,width()))