Detect failure from event_init() or event_base_new_with_config()

changes/bug4457

@@ -5,4 +5,5 @@
       bug 4457; workaround for Libevent versions 2.0.1-alpha through
       2.0.15-stable.
 
+    - Detect failure to initialize Libevent. Better detection for bug 4457.
 

src/common/compat_libevent.c

@@ -179,6 +179,7 @@ tor_libevent_initialize(void)
 #ifdef HAVE_EVENT2_EVENT_H
   {
     struct event_config *cfg = event_config_new();
+    tor_assert(cfg);
 
     /* In 0.2.2, we don't use locking at all.  Telling Libevent not to try to
      * turn it on can avoid a needless socketpair() attempt.
@@ -193,6 +194,11 @@ tor_libevent_initialize(void)
   the_event_base = event_init();
 #endif
 
+  if (!the_event_base) {
+    log_err(LD_GENERAL, "Unable to initialize Libevent: cannot continue.");
+    exit(1);
+  }
+
 #if defined(HAVE_EVENT_GET_VERSION) && defined(HAVE_EVENT_GET_METHOD)
   /* Making this a NOTICE for now so we can link bugs to a libevent versions
    * or methods better. */