Use new QByteArray::assign() in QTlsBackendOpenSSL::dhParametersFromPem()

QBA::assign() re-uses existing unshared capacity(), if any, and is
therefore potentially more efficient than = QByteArray(.,.) (and never
slower).

Pick-to: 6.6
Task-number: QTBUG-106201
Change-Id: I2c45aa268c4c06396e9d7e0490666a13d8cfd532
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/plugins/tls/openssl/qssldiffiehellmanparameters_openssl.cpp

@@ -139,7 +139,7 @@ int QTlsBackendOpenSSL::dhParametersFromPem(const QByteArray &pem, QByteArray *d
             const int len = q_i2d_DHparams(dh, reinterpret_cast<unsigned char **>(&buf));
             const auto freeBuf = qScopeGuard([&] { q_OPENSSL_free(buf); });
             if (len > 0)
-                *data = QByteArray(buf, len);
+                data->assign({buf, len});
             else
                 return DHParams::InvalidInputDataError;
         } else {