Don't call Libevent's event_base_free() on NULL.

It doesn't crash, but it produces a warning.

Fixes bug 24933; bugfix on 322abc030e53c7e84ca9f22a47b2965f2. Bug
not in any released Tor.

src/common/compat_libevent.c

@@ -243,7 +243,8 @@ tor_init_libevent_rng(void)
 void
 tor_libevent_free_all(void)
 {
-  event_base_free(the_event_base);
+  if (the_event_base)
+    event_base_free(the_event_base);
   the_event_base = NULL;
 }