Fix UTF8 encoding fallback index (dotnet/coreclr#18387)

Fix UTF8 encoding fallback index (dotnet/coreclr#18387)

* Fix UTF8 encoding fallback index

When falling back in UTF8 decoding, we’ll call the DecoderFallbackBuffer.InternalFallback method which takes a pointer to the source bytes pointer which point to where we have progressed inside the source bytes. The problem is before calling DecoderFallbackBuffer.InternalFallback we call GetBytesUnknown which can adjust the source byte pointer and DecoderFallbackBuffer.InternalFallback need to be called with the pointer before the adjustment. The fix is simply storing the original pointer value of the source byte pointer before calling GetBytesUnknown and then pass the stored original pointer to DecoderFallbackBuffer.InternalFallback instead of the adjusted source byte pointer

* Fix the comment

Commit migrated from https://github.com/dotnet/coreclr/commit/d093d9a9c83182f92a84c05af89e4859611d095a