Add compatibility to build OpenVPN with AWS-LC.

Add compatibility to build OpenVPN with AWS-LC.

Additional context from PR on Github about changes in ssl_openssl.c
around line 1900:
This change addresses a subtle behavioral difference between AWS-LC
and OpenSSL regarding object ownership semantics in
SSL_CTX_set_client_CA_list(ctx->ctx, cert_names).

OpenSSL Behavior:
Stores a reference to the provided cert_names stack
cert_names remains valid after SSL_CTX_set_client_CA_list

AWS-LC Behavior:
Creates a copy of the parameter cert_names (which is a stack of type
X509_NAME) and converts it to a stack of CRYPTO_BUFFER (how we internally
represent X509_NAME, it's an opaque byte string).  Then frees the original
passed in cert_names.
After SSL_CTX_set_client_CA_list, cert_names no longer points to valid
memory.

The proposed changes reorder operations to getting the size of the
stack before the set operation as opposed to after the set operation.
No operations between the setter and stack size check modify
cert_names. Therefore, the logical outcome should remain the same
- and this would also handle the subtle behavioral difference in
AWS-LC.

Github: closes OpenVPN/openvpn#672
Signed-off-by: Shubham Mittal <smittals@amazon.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20250128220932.2113-1-smittals@amazon.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg30682.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>