Use size of target buffer for allocation

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/13055)

ssl/ssl_sess.c

@@ -110,7 +110,7 @@ SSL_SESSION *ssl_session_dup(const SSL_SESSION *src, int ticket)
 {
     SSL_SESSION *dest;
 
-    dest = OPENSSL_malloc(sizeof(*src));
+    dest = OPENSSL_malloc(sizeof(*dest));
     if (dest == NULL) {
         goto err;
     }