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
cleanup.
Let's allow that use case.
Fixes#5255Closes#5257
Reported-by: davidedec on github
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ff7a3107ed