[Client] Ensure physics actor is valid in World::setOnGround()

Do the same in World::setInertialForce()

apps/openmw/mwworld/worldimp.cpp

@@ -1715,9 +1715,13 @@ namespace MWWorld
     void World::setInertialForce(const Ptr& ptr, const osg::Vec3f &force)
     {
         MWPhysics::Actor *actor = mPhysics->getActor(ptr);
+
+        if (actor != nullptr)
+        {
             actor->setOnGround(false);
             actor->setInertialForce(force);
         }
+    }
     /*
         End of tes3mp addition
     */
@@ -1731,8 +1735,12 @@ namespace MWWorld
     void World::setOnGround(const Ptr& ptr, bool onGround)
     {
         MWPhysics::Actor* actor = mPhysics->getActor(ptr);
+
+        if (actor != nullptr)
+        {
             actor->setOnGround(onGround);
         }
+    }
     /*
         End of tes3mp addition
     */