Fix physics to not trigger Bullet assert in Debug

When physics attempts to move by a very small amount, precision losses
caused Bullet to trigger an assert in debug from normalizing a zero
length vector.

apps/openmw/mwworld/physicssystem.cpp

@@ -299,6 +299,8 @@ namespace MWWorld
                     continue; // velocity updated, calculate nextpos again
                 }
 
+                if(!newPosition.positionCloses(nextpos, 0.00000001))
+                {
                     // trace to where character would go if there were no obstructions
                     tracer.doTrace(colobj, newPosition, nextpos, engine);
 
@@ -309,6 +311,20 @@ namespace MWWorld
                         remainingTime *= (1.0f-tracer.mFraction); // FIXME: remainingTime is no longer used so don't set it?
                         break;
                     }
+                }
+                else
+                {
+                    // The current position and next position are nearly the same, so just exit.
+                    // Note: Bullet can trigger an assert in debug modes if the positions
+                    // are the same, since that causes it to attempt to normalize a zero
+                    // length vector (which can also happen with nearly identical vectors, since
+                    // precision can be lost due to any math Bullet does internally). Since we
+                    // aren't performing any collision detection, we want to reject the next
+                    // position, so that we don't slowly move inside another object.
+                    remainingTime *= (1.0f-tracer.mFraction); // FIXME: remainingTime is no longer used so don't set it?
+                    break;
+                }
+
 
                 Ogre::Vector3 oldPosition = newPosition;
                 // We hit something. Try to step up onto it. (NOTE: stepMove does not allow stepping over)