TLS backend SecureTransport: properly clear SSLContextRef

Simply calling SSLClose on context is not enough. Properly clear
the context by calling 'reset' member-function.

Change-Id: Ic6a32f6e4193bb19809d1cce184651b78d6273a7
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 6128a92e07c0c66c29445c2d79c3b6bdbf8efcda)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/tls/securetransport/qtls_st.cpp

@@ -373,6 +373,7 @@ void TlsCryptographSecureTransport::disconnectFromHost()
     if (context) {
         if (!shutdown) {
             SSLClose(context);
+            context.reset(nullptr);
             shutdown = true;
         }
     }