Skip disabled actuators in mjd_actuator_vel.

Fixes a bug where the derivative of the actuator force with respect to the generalized velocities (used in implicit and implicitfast integrators) was failing to take into account disabled actuators.

Fixes #1838

PiperOrigin-RevId: 657193960
Change-Id: Id8c0ab863a39e2a01cd2703774f460e9731b4807

src/engine/engine_derivative.c

@@ -834,6 +834,11 @@ void mjd_actuator_vel(const mjModel* m, mjData* d) {
 
   // process actuators
   for (int i=0; i < m->nu; i++) {
+    // skip if disabled
+    if (mj_actuatorDisabled(m, i)) {
+      continue;
+    }
+
     mjtNum bias_vel = 0, gain_vel = 0;
 
     // affine bias