Prevent use of uninitialized variable.

Problem: `wait()` wasn't safe: if `receive()` failed,
if would use an unitialized message.
Solution: Make sure received didnt fail.

Problem: If `receive()` was interrupt (EINTR) actor
constructor would use a non-initialized signal, causing the
assertion to fire.
Solution: Replace call to `receive()` by `wait()` in actor constructor.