Sanitize emg offset indices

[zen-juen]

#524

Checklist

Here are some things to check before creating the PR. If you encounter any issues, do let us know :)

• I have read the CONTRIBUTING file.
• My PR is targetted at the dev branch (and not towards the master branch).
• I ran the CODE CHECKS on the files I added or modified and fixed the errors.
• I have added the newly added features to News.rst (if applicable)

[codecov-commenter]

Codecov Report

Merging #526 (917ce9e) into dev (8e71458) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #526      +/-   ##
==========================================
- Coverage   85.34%   85.32%   -0.02%     
==========================================
  Files         199      199              
  Lines        9771     9772       +1     
==========================================
- Hits         8339     8338       -1     
- Misses       1432     1434       +2     

Impacted Files	Coverage Δ
neurokit2/emg/emg_activation.py	87.32% <100.00%> (+0.08%)	⬆️
neurokit2/rsp/rsp_simulate.py	94.64% <0.00%> (-5.36%)	⬇️
neurokit2/ecg/ecg_findpeaks.py	93.92% <0.00%> (+0.17%)	⬆️
neurokit2/signal/signal_fixpeaks.py	97.70% <0.00%> (+0.76%)	⬆️
neurokit2/eda/eda_eventrelated.py	100.00% <0.00%> (+2.04%)	⬆️

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