Fix crash bugs when a bufferevent's eventcb is not set. In many places throughout the code, we called _bufferevent_run_eventcb without checking whether the eventcb was actually set. This would work fine when the bufferevent's callbacks were deferred, but otherwise the code would segfault. Strangely, we always remembered to check before calling the _bufferevent_run_{read,write}cb functions. To prevent similar errors in the future, all of _buferevent_run_{read,write,event}cb now check to make sure the callback is actually set before invoking or deferring the callback. This patch also removes the now-redundant checks for {read,write}cb.