AC_AttControl: do not limit rate if ATC_ACCEL_MAX param is zero

libraries/AC_AttitudeControl/AC_AttitudeControl.cpp

@@ -480,8 +480,12 @@ float AC_AttitudeControl::input_shaping_angle(float error_angle, float smoothing
     float ang_vel = sqrt_controller(error_angle, smoothing_gain, accel_max);
 
     // Acceleration is limited directly to smooth the beginning of the curve.
-    float delta_ang_vel = accel_max * _dt;
-    return constrain_float(ang_vel, target_ang_vel-delta_ang_vel, target_ang_vel+delta_ang_vel);
+    if (accel_max > 0) {
+        float delta_ang_vel = accel_max * _dt;
+        return constrain_float(ang_vel, target_ang_vel-delta_ang_vel, target_ang_vel+delta_ang_vel);
+    } else {
+        return ang_vel;
+    }
 }
 
 // limits the acceleration and deceleration of a velocity request