BUG/MEDIUM: ssl: fix error path on generate-certificates

BUG/MEDIUM: ssl: fix error path on generate-certificates

It was reported by Przemyslaw Bromber that using the "generate-certificates"
option combined with AWS-LC would crash HAProxy when a request is done with a
SNI longer than 64 bytes.

The problem is that the certificate is generated with a CN greater than 64
bytes which results in ssl_sock_do_create_cert() returning NULL. This
NULL value being passed to SSL_set_SSL_CTX.

With OpenSSL, passing a NULL SSL_CTX does not seem to be an issue as it
would just ignore it.

With AWS_LC, passing a NULL seems to crash the function. This was
reported to upstream AWS-LC and fixed in patch 7487ad1dcd8
https://github.com/aws/aws-lc/pull/2946.

This must be backported in every branches.