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;
     }
   }
 }

test/engine/engine_collision_gjk_test.cc

@@ -1397,7 +1397,7 @@ TEST_F(MjGjkTest, BoxBoxMultiCCD15) {
   mjtNum dist;
   int ncons = Penetration(status, dist, dir, pos, model, data, g1, g2, 0, 4);
 
-  ASSERT_EQ(ncons, sizeof(mjtNum) == 8 ? 4 : 0);
+  ASSERT_EQ(ncons, 4);
 }
 
 TEST_F(MjGjkTest, SmallBoxMesh) {