[Fuzzing] [buyaa-n] Fix bug in validating unused bits

[Job](https://mihubot.xyz/runtime-utils/95ce60861607421b811dd060658b0e98) completed in 1 hour 38 minutes.
https://github.com/dotnet/runtime/pull/106771


Artifacts:
- [Base64UrlFuzzer-inputs.zip](https://runtimeutils.blob.core.windows.net/artifacts/95ce60861607421b811dd060658b0e98/Base64UrlFuzzer-inputs.zip) (624 KB)

