In order to track what was discussed in #78 I open this issue tracker.

@wjwwood suggested that in the future it might become necessary/useful to implement functions in the style rcl_x_is_valid .

An example would be: bool rcl_service_is_valid(rcl_service_t * service) which would check if this service is valid. (Perhaps check if it's not null, check if it was initialized, and so on)