Minor correctness and warning fixes

[ehntoo]

When compiling with -O3 under clang on an Apple Silicon macbook, 64-bit CRCs were failing the test functions:

CRC-40/GSM          : d4164fc646       (OK)   : d4164fc646       (OK)
CRC-64              : d4164fc646       (FAIL) : d4164fc646       (FAIL)
CRC-64/WE           : d4164fc646       (FAIL) : d4164fc646       (FAIL)
CRC-64/XZ           : 995dc9bbdf1939fa (OK)   : fffffffc3d327d15 (FAIL)

This PR patches up the mask generation to avoid the undefined behavior that was causing the issue.

CRC-40/GSM          : d4164fc646       (OK)   : d4164fc646       (OK)
CRC-64              : 6c40df5f0b497347 (OK)   : 6c40df5f0b497347 (OK)
CRC-64/WE           : 62ec59e3f1a4f00a (OK)   : 62ec59e3f1a4f00a (OK)
CRC-64/XZ           : 995dc9bbdf1939fa (OK)   : 995dc9bbdf1939fa (OK)