E2S11: Fix 0.9.9 regression causing Hekuba's keep to crumble down over time

This had the side-effect of killing him prematurely after a random
number of turns, rendering the scenario impossible to complete unless
Ivyel is around to get killed by the portal logic before/afterwards.

Introduced by commit 22c401f7e4209aa0fdbc9186e4ec133a489d53ae due to a
terrain change that caused a filter to not match Hekuba's keep anymore.