Sni csharp wrapper patch

[gasbytes]

Description

Fixes zd#17990

Testing

server:

mcs wolfSSL_CSharp/wolfSSL.cs wolfSSL_CSharp/X509.cs \
wolfSSL-TLS-Server/wolfSSL-TLS-Server.cs -OUT:server.exe

client:

mcs wolfSSL_CSharp/wolfSSL.cs wolfSSL_CSharp/X509.cs \
wolfSSL-TLS-Client/wolfSSL-TLS-Client.cs -OUT:client.exe

Checklist

• [x] added tests
• [ ] updated/added doxygen
• [ ] updated appropriate READMEs
• [ ] Updated manual and documentation