The caller and listener callback functions should be:

• fully allowed to be changed, while the socket isn't connected or listening (including setting it to NULL)
• not allowed to be changed, when the socket is connected, listening, or even broken

Currently only non-NULL callback function is accepted, although regardless of the current socket state. This function shall not be allowed to be altered when the socket is in the state that makes it involved in some ongoing activities, whereas there shouldn't be any problem to set it to NULL at the moment when it's not (yet).