Fix cuda-memcheck errors for some libcudf functions

[davidwendt]

Reference #8883

This PR fixes cuda-memcheck errors found in STRINGS_TEST TEXT_TEST, CSV_TEST and GROUPBY_TEST. All of these were reading outside the bounds of a device memory buffer. Extra checking was added to the code logic to ensure bytes are not read past the end of given buffer.

These were found a checked using cuda-memcheck gtests/[test-executable] --rmm_mode=cuda since the default pool mode will render all or most device memory as valid readable.

[davidwendt]

rerun tests

[codecov]

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[davidwendt]

rerun tests

[vuule]

@gpucibot merge