Race condition on event_debug_mode_too_late.

[Yang Gao]

Hi,

In one of our tests, we saw a race condition on event_debug_mode_too_late.
We are using 2.0.21 stable.

In the test, one thread has on top of the stack
#0 event_assign
#1 event_new
#2 ... omitted

and another is doing
#0 event_add_internal
#1 event_add
#2 ... omitted

Setting macro _EVENT_DISABLE_DEBUG_MODE fixes the problem.

A google search shows this might be similar to reports here:
http://archives.seul.org/libevent/users/Nov-2010/msg00058.html
and here:
http://sourceforge.net/p/levent/bugs/182/

Thanks.

Yang