Fix memcheck read error in compound segmented reduce

[davidwendt]

Description

Fixes an out-of-bounds memory read in the compound segmented reduction logic. The number of segments was computed incorrectly causing an extra read passed the end of the valid-counts vector. This was found by running compute-sanitizer test on the reductions gtests as follows:

compute-sanitizer --tool memcheck --demangle full gtests/REDUCTIONS_TEST --rmm_mode=cuda

The number of segments is 1 less than the number of offsets.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

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Patch has no changes to coverable lines.

Additional details and impacted files

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[bdice]

Thanks for the fix!

[davidwendt]

/merge