multi-ssl: reset the SSL backend on `Curl_global_cleanup()`

When cURL is compiled with support for multiple SSL backends, it is
possible to configure an SSL backend via `curl_global_sslset()`, but
only *before* `curl_global_init()` was called.

If another SSL backend should be used after that, a user might be
tempted to call `curl_global_cleanup()` to start over. However, we did
not foresee that use case and forgot to reset the SSL backend in that

Let's allow that use case.

Fixes #5255
Closes #5257
Reported-by: davidedec on github
Signed-off-by: Johannes Schindelin <>
diff --git a/lib/vtls/vtls.c b/lib/vtls/vtls.c
index badd93d..f1b5252 100644
--- a/lib/vtls/vtls.c
+++ b/lib/vtls/vtls.c
@@ -174,6 +174,9 @@
   return Curl_ssl->init();
+#if defined(CURL_WITH_MULTI_SSL)
+static const struct Curl_ssl Curl_ssl_multi;
 /* Global cleanup */
 void Curl_ssl_cleanup(void)
@@ -181,6 +184,9 @@
   if(init_ssl) {
     /* only cleanup if we did a previous init */
+#if defined(CURL_WITH_MULTI_SSL)
+    Curl_ssl = &Curl_ssl_multi;
     init_ssl = FALSE;