Remove check against passed bufferevent in bufferevent_socket_connect()

Remove check against passed bufferevent in bufferevent_socket_connect()

It is a bit confusing to check against the result of EVUTIL_UPCAST(),
because it can return not NULL for NULL pointers, but for even though
with bufferevent we are fine (because bufferevent is the first field in
bufferevent_private), there are no checks for "bufev" in bufferevent's
API, so just remove it to make it generic.

Fixes: #542
Signed-off-by: Ivan Maidanski <i.maidanski@samsung.com>
Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>