* 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.
* add freezing and unfreezing of peers
* add rejoin packet
* revise handling of temporary invited connections
* rename "peer kill" packet to "peer leave" packet
* test rejoining in conference test
* use custom clock in conference test
This forces all the loop bodies to be executed at least once, which is
harmless since it just means one more tox event loop iteration. This
reduces the jitter we see in coverage measurements, which is partially
caused by loops sometimes being entered and sometimes not (because their
condition happens to randomly already be true).
