Make SSL_is_server() accept a const SSL

Fixes #1526
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3360)

include/openssl/ssl.h

@@ -1996,7 +1996,7 @@ int SSL_set_block_padding(SSL *ssl, size_t block_size);
 # endif
 
 __owur int SSL_session_reused(SSL *s);
-__owur int SSL_is_server(SSL *s);
+__owur int SSL_is_server(const SSL *s);
 
 __owur __owur SSL_CONF_CTX *SSL_CONF_CTX_new(void);
 int SSL_CONF_CTX_finish(SSL_CONF_CTX *cctx);

ssl/ssl_lib.c

@@ -4015,7 +4015,7 @@ int SSL_session_reused(SSL *s)
     return s->hit;
 }
 
-int SSL_is_server(SSL *s)
+int SSL_is_server(const SSL *s)
 {
     return s->server;
 }