tcp-ssl: only call event-cb on error if it's not nil
tcp-ssl: only call event-cb on error if it's not nil

If event-cb is nil, the existing code raises an error because event-cb is
nil. Let's prevent this.

Signed-off-by: Julien Danjou <5c682c2d1ec4073e277f9ba9f4bdf07e5794dabe@danjou.info>

orthecreedence/cl-async,orivej/cl-async,Drooids/cl-async