output drivers: init SmartLock after exit_and_cleanup

This fixes an invalid memory access when exiting the module:
exit_and_cleanup destroys the object, but lock_guard is destructed after
and accesses the lock.

src/drivers/linux_pwm_out/linux_pwm_out.cpp

@@ -126,8 +126,6 @@ bool LinuxPWMOut::updateOutputs(bool stop_motors, uint16_t outputs[MAX_ACTUATORS
 
 void LinuxPWMOut::Run()
 {
-	SmartLock lock_guard(_lock);
-
 	if (should_exit()) {
 		ScheduleClear();
 		_mixing_output.unregister();
@@ -136,6 +134,8 @@ void LinuxPWMOut::Run()
 		return;
 	}
 
+	SmartLock lock_guard(_lock);
+
 	perf_begin(_cycle_perf);
 	perf_count(_interval_perf);