Add an assertion to tor_libevent_get_base()

Closes ticket 18241.

changes/assert_event_base

@@ -0,0 +1,5 @@
+  o Minor features (robustness):
+    - Exit immediately with an error message if the code attempts to
+      use libevent without having initialized it. This should resolve
+      some frequently-made mistakes in our unit tests. Closes ticket
+      18241.

src/common/compat_libevent.c

@@ -247,6 +247,7 @@ tor_libevent_initialize(tor_libevent_cfg *torcfg)
 MOCK_IMPL(struct event_base *,
 tor_libevent_get_base, (void))
 {
+  tor_assert(the_event_base != NULL);
   return the_event_base;
 }