Update x_norm in tetrahedron case in GJK.

This check is never called due to the fallback to gjkIntersection, however it's needed for debugging when disabling the fallback.

PiperOrigin-RevId: 792623394
Change-Id: I7178d9afae697bd22984688e89fd1865d3c6a37f

src/engine/engine_collision_gjk.c

@@ -278,6 +278,7 @@ static void gjk(mjCCDStatus* status, mjCCDObj* obj1, mjCCDObj* obj2) {
 
     // we have a tetrahedron containing the origin so return early
     if (n == 4) {
+      x_norm = 0;
       break;
     }
   }