Remember the centre of Gravitas when its target dies.

On most parts of the map, if one attractee collided with and killed the
target, further attactees wouldn't be moved, because the dead target was
too far away.  That's a bit weird because, notionally, they're all being
attracted at the same time.

Even worse, if we happened to be close to (0,0), the remaining monsters
would be sucked to the northwest, where the spirits of the departed
reside.

crawl-ref/source/spl-transloc.cc

@@ -1002,19 +1002,20 @@ void attract_actor(const actor* agent, actor* victim, const coord_def pos,
 bool fatal_attraction(actor *victim, actor *agent, int pow)
 {
     bool affected = false;
-    for (actor_near_iterator ai(victim->pos(), LOS_NO_TRANS); ai; ++ai)
+    const auto pos = victim->pos(); // in case it dies
+    for (actor_near_iterator ai(pos, LOS_NO_TRANS); ai; ++ai)
     {
         if (*ai == victim || *ai == agent || ai->is_stationary())
             continue;
 
-        const int range = (victim->pos() - ai->pos()).rdist();
+        const int range = (pos - ai->pos()).rdist();
         const int strength =
             min(4, (pow / 10) / (range*range));
         if (strength <= 0)
             continue;
 
         affected = true;
-        attract_actor(agent, *ai, victim->pos(), pow, strength);
+        attract_actor(agent, *ai, pos, pow, strength);
     }
 
     return affected;