More saveguards and logging in get_ultimate_keys

* src/parsecontroller.cpp (ParseController::get_ultimate_keys):
Add more debug output and saveguards.

From the logs in a bugreport it looks very much like we
crash somewhere in between starting the keylisting and
finishing it.

Checking for invalid key or if a uid has an id makes
sense, but might not be the crash.

• GnuPG-Bug-Id: T3857