Don't attempt to log messages to a controller from a worker thread.

This patch adds a function to determine whether we're in the main
thread, and changes control_event_logmsg() to return immediately if
we're in a subthread.  This is necessary because otherwise we will
call connection_write_to_buf, which modifies non-locked data
structures.

Bugfix on 0.2.0.x; fix for at least one of the things currently
called "bug 977".

src/or/control.c

@@ -3367,6 +3367,11 @@ control_event_logmsg(int severity, uint32_t domain, const char *msg)
 {
   int event;
 
+  /* Don't even think of trying to add stuff to a buffer from a cpuworker
+   * thread. */
+  if (! in_main_thread())
+    return;
+
   if (disable_log_messages)
     return;