Do not try to shut down the event loop when it is not initialized.

Doing so caused us to crash in some unusual circumstances, such as
using --verify-config to verify a configuration that failed during
the options_act() stage.

Fixes bug 32407; bugfix on 0.3.3.1-alpha.

src/core/mainloop/mainloop.c

@@ -774,6 +774,10 @@ tor_shutdown_event_loop_and_exit(int exitcode)
   main_loop_should_exit = 1;
   main_loop_exit_value = exitcode;
 
+  if (! tor_libevent_is_initialized()) {
+    return; /* No event loop to shut down. */
+  }
+
   /* Die with an assertion failure in ten seconds, if for some reason we don't
    * exit normally. */
   /* XXXX We should consider this code if it's never used. */