Fix a traceback when closing a blocked connection "immediately".

When we close a connection via connection_close_immediately, we kill
its events immediately. But if it had been blocked on bandwidth
read/write, we could try to re-add its (nonexistent) events later
from connection_bucket_refill -- if we got to that callback before
we swept the marked connections.

Fixes bug 24167.  Fortunately, this hasn't been a crash bug since we
introduced connection_check_event in 0.2.9.10, and backported it.

This is a bugfix on commit 89d42291, I believe, which
appeared in Tor 0.1.0.1-rc.

changes/bug24167

+  o Minor bugfixes (network layer):
+    - When closing a connection via close_connection_immediately(), we
+      mark it as "not blocked on bandwidth", to prevent later calls
+      from trying to unblock it, and give it permission to read. This
+      fixes a backtrace warning that can happen on relays under various
+      circumstances. Fixes bug 24167; bugfix on 0.1.0.1-rc.
+

src/or/connection.c

@@ -721,6 +721,10 @@ connection_close_immediate(connection_t *conn)
 
   connection_unregister_events(conn);
 
+  /* Prevent the event from getting unblocked. */
+  conn->read_blocked_on_bw =
+    conn->write_blocked_on_bw = 0;
+
   if (SOCKET_OK(conn->s))
     tor_close_socket(conn->s);
   conn->s = TOR_INVALID_SOCKET;