Fix crash bugs when a bufferevent's eventcb is not set.

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.