Fix use-after-free in SSLClient destructor with mbedTLS (Fix #2492)

SSLClient::~SSLClient() freed the TLS context before shutting down
the SSL session. mbedTLS sessions hold a raw pointer into the
context's mbedtls_ssl_config, so a live keep-alive session's
close_notify would read freed memory. Shut down the session first,
then free the context.

Add a regression test that destructs an SSLClient while a keep-alive
mbedTLS session is still open.

httplib.h

@@ -16107,11 +16107,18 @@ inline bool SSLServer::update_certs_pem(const char *cert_pem,
 
 // SSL HTTP client implementation
 inline SSLClient::~SSLClient() {
-  if (ctx_) { tls::free_context(ctx_); }
   // Make sure to shut down SSL since shutdown_ssl will resolve to the
   // base function rather than the derived function once we get to the
   // base class destructor, and won't free the SSL (causing a leak).
+  // This must happen before the context is freed below: some backends
+  // (e.g. mbedTLS) have the SSL session borrow a raw pointer into the
+  // context, so freeing the context first leaves close_notify reading
+  // freed memory.
   shutdown_ssl_impl(socket_, true);
+  if (ctx_) {
+    tls::free_context(ctx_);
+    ctx_ = nullptr;
+  }
 }
 
 inline bool SSLClient::is_valid() const { return ctx_ != nullptr; }