Blob::offset() indexing

[hkxIron]

Fix bug issue #5964 : change CHECK_LE to CHECK_LT in function offset.

[hkxIron]

Thanks.

[hkxIron]

@shaibagon , run unit test failed.
F1101 15:51:51.257525 12917 blob.hpp:158] Check failed: c < channels() (1 vs. 1)

[shaibagon]

@hkxIron It seems like pooling layer's CPU code uses offset() to compute a step size inside the blob, This step is taken at the end of a loop, so it might be the case that the step is computed, but when it falls outside the Blob it's the end of the loop and the memory is not accessed.
I guess this PR will have to deal/fix this as well.
I would try and pre-compute these steps in advance (i.e., calling offset(0,1) once outside the loop and saving the value and then re-using it instead of calling offset(0,1) multiple times.

This is why we have automatic tests: you can never know what a small change can affect :(

[Noiredd]

I would also add #792 for reference - I think that PR was more comprehensive - and #3420 for the conversation.