Ensure that vent->smeth != NULL before we call vent->smeth()

We can end up with a NULL SSL_METHOD function if a method has been
disabled. If that happens then we shouldn't call vent->smeth().
Reviewed-by: NRich Salz <rsalz@openssl.org>

ssl/statem/statem_lib.c

@@ -1053,7 +1053,7 @@ int ssl_choose_server_version(SSL *s, CLIENTHELLO_MSG *hello)
                  vent->version != 0 && vent->version != (int)candidate_vers;
                  ++vent)
                 ;
-            if (vent->version != 0) {
+            if (vent->version != 0 && vent->smeth != NULL) {
                 const SSL_METHOD *method;
 
                 method = vent->smeth();