TSG S5: fix an unlikely scenario outcome

In case the player goes south and runs away fromt he bandits, and defeats
the lich first, then the choice to side or not with the bandits would
happen very late.
In case he allied with them and the other undead leader still lived, he
could end the scenario still by triggering "enemies defeated". If they were
already defeated, the scenario could not end anymore. Furthermore, the
storytext doesn't make sense anymore, at least the ending dilogue would
miss,
In case of siding with the elves one can still win by defeating Urza Afalas.
and the story still makes sense,

To solve this, in this special situation the player has no choice anymore,
the path of the elves is chosen.

(the diff may be hard to read; a good part has been wrappend with an [if])

[ci skip]

(cherry-picked from commit 5dc80e7)