fixed a segmentation fault when entering a cell which contains someone already attacking you

apps/openmw/mwmechanics/aicombat.cpp

@@ -229,6 +229,9 @@ namespace MWMechanics
         osg::Vec3f vAimDir = MWBase::Environment::get().getWorld()->aimToTarget(actor, target);
         float distToTarget = MWBase::Environment::get().getWorld()->getHitDistance(actor, target);
 
+        if (!currentAction)
+            return;
+
         storage.mReadyToAttack = (currentAction->isAttackingOrSpell() && distToTarget <= rangeAttack);
         
         // can't fight if attacker can't go where target is.  E.g. A fish can't attack person on land.