Fix memcheck read error in libcudf contiguous_split

[davidwendt]

Reference #8883

The cudf::contiguous_split was failing on memcheck using the compute-sanitizer with a 4-byte out-of-bounds read. This was traced to the copy_buffer device function that was reading 1 past the end of the input buffer when performing a value-shift. The ternary check was incorrectly protecting the out-of-bounds read. The logic is corrected by this PR.

Also, I fixed some const removal casts from the same source file by adding appropriate const qualifiers to the input data variables.

[codecov]

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[davidwendt]

@gpucibot merge