* Use-after-free because we free network before dht in one case.
* Various unchecked allocs in tests (not so important).
* We used to not check whether ping arrays were actually allocated in DHT.
* `ping_kill` and `ping_array_kill` used to crash when passing NULL.
Also:
* Added an assert in all public API functions to ensure tox isn't NULL.
The error message you get from that is a bit nicer than "Segmentation
fault" when clients (or our tests) do things wrong.
* Decreased the sleep time in iterate_all_wait from 20ms to 5ms.
Everything seems to still work with 5ms, and this greatly decreases
the amount of time spent per test run, making oomer run much faster.
