BUG/MEDIUM: checks: Don't bother subscribing if we have a connection error.

BUG/MEDIUM: checks: Don't bother subscribing if we have a connection error.

In __event_srv_chk_r() and __event_srv_chk_w(), don't bother subscribing
if we're waiting for a handshake, but we had a connection error. We will
never be able to send/receive anything on that connection anyway, and
the conn_stream is probably about to be destroyed, and we will crash if
the tasklet is waken up.
I'm not convinced we need to subscribe here at all anyway, but I'd rather
modify the check code as little as possible.

This should be backported to 1.9.