Fix bug in validating unused bits

[buyaa-n]

Fix bug in validating unused bits found with Base64Url fuzzer

The input that failed the fuzzer is Base64Url.IsValid("SM=") which supposed to return false, invalid input

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-memory
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[MihaZupan]

Looks like there are still some unhandled cases: MihuBot/runtime-utils#562

// Base64UrlFuzzer
System.Exception: Expected=Done Actual=InvalidData
   at DotnetFuzzing.Assert.<Equal>g__Throw|0_0[T](T expected, T actual) in D:\a\runtime-utils\runtime-utils\Runner\runtime\src\libraries\Fuzzing\DotnetFuzzing\Assert.cs:line 18
   at DotnetFuzzing.Assert.Equal[T](T expected, T actual) in D:\a\runtime-utils\runtime-utils\Runner\runtime\src\libraries\Fuzzing\DotnetFuzzing\Assert.cs:line 14
   at DotnetFuzzing.Fuzzers.Base64UrlFuzzer.FuzzTarget(ReadOnlySpan`1 bytes) in D:\a\runtime-utils\runtime-utils\Runner\runtime\src\libraries\Fuzzing\DotnetFuzzing\Fuzzers\Base64UrlFuzzer.cs:line 109
   at SharpFuzz.Fuzzer.LibFuzzer.Run(ReadOnlySpanAction action, Boolean ignoreExceptions)
==8932== ERROR: libFuzzer: deadly signal
NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
MS: 1 CrossOver-; base unit: 122a96fbeb0d3493dcb52f34d0b7bfecbec11036
0x73,0xa,0x45,0x73,0xa,0xa,0x3d,0x3d,
s\012Es\012\012==
artifact_prefix='./'; Test unit written to Base64UrlFuzzer-artifact-2
Base64: cwpFcwoKPT0=

[MihaZupan]

@MihuBot fuzz Base64Url -combineWith #106509

[buyaa-n]

@MihuBot fuzz Base64Url -combineWith #106509

Its really cool that we can run a fuzzer with a given PR, thanks!

[buyaa-n]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0: https://github.com/dotnet/runtime/actions/runs/10691347585