[scudo/standalone] Fix undefined behavior in checksum test

1U has type unsigned int, and << of 32 or more is undefined behavior.
Use the proper type in the lhs of the shift.

Reviewed By: cryptoad

Differential Revision: https://reviews.llvm.org/D87973

GitOrigin-RevId: f5fa5b9fe3b02f26b38612f96cba19aceccbc2a3
1 file changed