LibTLS: Avoid unnecessary HashMap copies, improve const-correctness

Author: https://github.com/BenWiederhake
Commit: 2bb2a7097d
Pull-request: https://github.com/SerenityOS/serenity/pull/18821
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/gmta ✅
Reviewed-by: https://github.com/kleinesfilmroellchen ✅

Userland/Libraries/LibTLS/HandshakeCertificate.cpp

@@ -78,7 +78,7 @@ ssize_t TLSv12::handle_certificate(ReadonlyBytes buffer)
 
             auto certificate = Certificate::parse_certificate(buffer.slice(res_cert, certificate_size_specific), false);
             if (!certificate.is_error()) {
-                m_context.certificates.append(certificate.value());
+                m_context.certificates.empend(certificate.value());
                 valid_certificate = true;
             } else {
                 dbgln("Failed to parse client cert: {}", certificate.error());