Fix memcheck error in distinct inner join

[PointKernel]

Description

Closes #15156
Fixes the invalid global read introduced by #14990 and simplifies the logic.

Checklist

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

[hyperbolic2346]

Where is the invalid global read?

[wence-]

Where is the invalid global read?

build_idx = static_cast<cudf::size_type>(found->second);

In the old code was invalid if found == hash_table.end(), but was executed unconditionally.

[PointKernel]

/merge