lib/tevent: a cleaner fix for be4ac227842530d484659f2db683453366326d8b segv

lib/tevent: a cleaner fix for be4ac227842530d484659f2db683453366326d8b segv

Revert 23abcd2318c69753aa2a144e1dc0f9cf9efdb705 and fix logic bug.

The current code loops through the event contexts, when it sees a different
one, it notifies the current one (ev) and updates ev to point to the new one.

This is dumb, because:
(1) ev starts as NULL, so this code crashes, and
(2) The final context will not be notified.

The correct fix for this is to update ev to the new one, then notify it.
Volker's fix works because we currently always have one event context.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>