Make -xcert work again.

When a certificate is prepended update the list pointer.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2628)

apps/s_cb.c

@@ -944,6 +944,7 @@ int args_excert(int opt, SSL_EXCERT **pexc)
             BIO_printf(bio_err, "%s: Error adding xcert\n", opt_getprog());
             goto err;
         }
+        *pexc = exc;
         exc->certfile = opt_arg();
         break;
     case OPT_X_KEY: