Fix errors and warnings in asserts

[oleksandr-pavlyk]

There was a typo in one of the assertions, and numerous comparisons between signed and unsigned types.
Recent fix to build_locally.py (#835) uncovered these, and this PR fixes the error and addresses the warnings,

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/836/index.html

[coveralls]

Coverage remained the same at 82.895% when pulling 458c4ea on fix-errors-and-warnings-in-asserts into 83bd76f on master.

[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞