Use mju_symmetrize in engine_derivative.c

PiperOrigin-RevId: 519111361
Change-Id: I3125538dee70eebca7e1e9e7830bde7863be956f

src/engine/engine_derivative.c

@@ -1209,13 +1209,7 @@ void mjd_ellipsoidFluid(const mjModel* m, mjData* d, int bodyid) {
 
     // make B symmetric if integrator is IMPLICITFAST
     if (m->opt.integrator == mjINT_IMPLICITFAST) {
-      for (int i=0; i<5; i++) {
-        for (int k=i+1; k<6; k++) {
-          mjtNum tmp = 0.5 * (B[6 * i + k] + B[6 * k + i]);
-          B[6 * i + k] = tmp;
-          B[6 * k + i] = tmp;
-        }
-      }
+      mju_symmetrize(B, B, 6);
     }
 
     if (mj_isSparse(m)) {