Zero out stale lambda coordinate in S3D in GJK.

PiperOrigin-RevId: 958414972
Change-Id: Ia21f8959a990c18b9d67959be5b2adeb1d3fa1dd

src/engine/engine_collision_gjk.c

@@ -677,6 +677,7 @@ static void S3D(mjtNum lambda[4], const mjtNum s1[3], const mjtNum s2[3],
       lambda[0] = lambda_2d[0];
       lambda[1] = lambda_2d[1];
       lambda[2] = lambda_2d[2];
+      lambda[3] = 0;
     }
   }
 }