Fixed potential double-readcb execution with openssl bufferevents.

Fixed potential double-readcb execution with openssl bufferevents.

the function do_read() will call SSL_read(), and if successful, will
call _bufferevent_run_readcb() before returning to consider_reading().

consider_reading() will then check SSL_pending() to make sure all
pending data has also been read. If it does not, do_read() is called
again.

The issue with this is the possibility that the function that is
executed by _bufferevent_run_readcb() called
bufferevent_disable(ssl_bev, EV_READ) before the second do_read(); In
this case, the users read callback is executed a second time. This is
potentially bad for applications that expect the bufferevent to stay
disabled until further notice. (this is why running openssl bufferevents
without DEFER_CALLBACKS has always been troublesome).