Avoid summon spawn if actor isn't in a cell (bug #5644)

CHANGELOG.md

@@ -55,6 +55,7 @@
     Bug #5603: Setting constant effect cast style doesn't correct effects view
     Bug #5611: Usable items with "0 Uses" should be used only once
     Bug #5622: Can't properly interact with the console when in pause menu
+    Bug #5644: Summon effects running on the player during game initialization cause crashes
     Feature #390: 3rd person look "over the shoulder"
     Feature #2386: Distant Statics in the form of Object Paging
     Feature #4894: Consider actors as obstacles for pathfinding

apps/openmw/mwmechanics/actors.cpp

@@ -1198,6 +1198,11 @@ namespace MWMechanics
             }
         }
 
+        // Summoned creature update visitor assumes the actor belongs to a cell.
+        // This assumption isn't always valid for the player character.
+        if (!ptr.isInCell())
+            return;
+
         bool hasSummonEffect = false;
         for (MagicEffects::Collection::const_iterator it = effects.begin(); it != effects.end(); ++it)
         {

apps/openmw/mwworld/worldimp.cpp

@@ -2182,6 +2182,8 @@ namespace MWWorld
 
     Ptr World::copyObjectToCell(const ConstPtr &object, CellStore* cell, ESM::Position pos, int count, bool adjustPos)
     {
+        if (!cell)
+            throw std::runtime_error("copyObjectToCell(): cannot copy object to null cell");
         if (cell->isExterior())
         {
             int cellX, cellY;