dns: Set the result to null in cancelLookup()

When lookupHost() is called the handle returned
from allocateHandle() might be a reused handle.

The delegate will be cleared, but the result might
still be there and the assert in lookupHost()
will fail:

```
assert(!m_lookups[handle].result);
```

source/eventcore/drivers/posix/dns.d

@@ -132,6 +132,7 @@ final class EventDriverDNS_GAI(Events : EventDriverEvents, Signals : EventDriver
 	{
 		if (!isValid(handle)) return;
 		m_lookups[handle].callback = null;
+		m_lookups[handle].result = null;
 		m_events.loop.m_waiterCount--;
 	}