don't update servos if mixer_mix_threadsafe() failed to run

src/modules/px4iofirmware/mixer.cpp

@@ -295,6 +295,9 @@ mixer_tick(void)
 		/* mix */
 		mixed = mixer_mix_threadsafe(&outputs[0], &r_mixer_limits);
 
+		/* if mixer_mix_threadsafe returns zero, it did nothing */
+		if (mixed != 0) {
+
 			/* the pwm limit call takes care of out of band errors */
 			pwm_limit_calc(should_arm, should_arm_nothrottle, mixed, r_setup_pwm_reverse, r_page_servo_disarmed,
 				       r_page_servo_control_min, r_page_servo_control_max, outputs, r_page_servos, &pwm_limit);
@@ -315,7 +318,7 @@ mixer_tick(void)
 
 				up_pwm_force_update();
 			}
-
+		}
 	}
 
 	/* set arming */