Guard against unsigned integer underflow

[tianyilim]

timestamps_50khz is an array of np.uint64. In the (rare) case that the difference between the two values in the array is 0, cnt underflows.

This fix checks for this case.

[sudoRicheek]

Hi @tianyilim. Nice catch! Thanks for the contribution.
However, this can't be merged as is, since I have updated the dataloader to support the millisecond to event index mapping, which is available for a lot of event datasets. This causes a minor conflict. Also, I have realized, we do not need the -1 there so you can directly check if cnt >= min_numevents_ctx (I will confirm this sometime).

If you want to modify the PR to make these changes, you are welcome to - and I will merge it!