Don't schedule excess_renegotiations_callback unless it's set

Partial fix for bug 4587; reported by "frosty_un".

src/common/tortls.c

@@ -1318,7 +1318,8 @@ tor_tls_got_client_hello(tor_tls_t *tls)
     }
 
     tls->got_renegotiate = 1;
-  } else if (tls->server_handshake_count > 2) {
+  } else if (tls->server_handshake_count > 2 &&
+             tls->excess_renegotiations_callback) {
     /* We got more than one renegotiation requests. The Tor protocol
        needs just one renegotiation; more than that probably means
        They are trying to DoS us and we have to stop them. We can't