sys/event: add assertion that event has a handler

[benpicco]

Contribution description

When an uninitialized event is added, we will dereference a NULL pointer. It can then be a bit tricky to find out which event caused this - so add an assertion when queuing the event that a callback is set.

Testing procedure

Issues/PRs references

[riot-ci]

Murdock results

:heavy_check_mark: PASSED

16c447afb1d1fbb540829aa43159aa235ac2cdcd gcoap: make use of event_callback_post()

Success	Failures	Total	Runtime
10193	0	10193	18m:19s

Artifacts

• Documentation preview

[benpicco]

A callback struct with a NULL handler does no harm in itself. Only when you try to add it to an event queue, things will blow up.