Skip raycasting for zero-length rays

Rays with no length can't intersect anything. This also prevents an
assertion triggering with Bullet built in Debug mode.

apps/openmw/mwphysics/physicssystem.cpp

@@ -264,6 +264,8 @@ namespace MWPhysics
 
     RayCastingResult PhysicsSystem::castRay(const osg::Vec3f &from, const osg::Vec3f &to, const MWWorld::ConstPtr& ignore, std::vector<MWWorld::Ptr> targets, int mask, int group) const
     {
+        if (from == to)
+            return RayCastingResult { false };
         btVector3 btFrom = Misc::Convert::toBullet(from);
         btVector3 btTo = Misc::Convert::toBullet(to);