mixer_multirotor: fix initialization of _outputs_prev

memset (http://www.cplusplus.com/reference/cstring/memset/) interprets
the value argument as unsigned char, so passing a float will not work.

src/lib/mixer/mixer_multirotor.cpp

 /****************************************************************************
  *
- *   Copyright (c) 2012-2016 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2012-2018 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -76,7 +76,9 @@ MultirotorMixer::MultirotorMixer(ControlCallback control_cb,
 	_rotors(_config_index[(MultirotorGeometryUnderlyingType)geometry]),
 	_outputs_prev(new float[_rotor_count])
 {
-	memset(_outputs_prev, _idle_speed, _rotor_count * sizeof(float));
+	for (unsigned i = 0; i < _rotor_count; ++i) {
+		_outputs_prev[i] = _idle_speed;
+	}
 }
 
 MultirotorMixer::~MultirotorMixer()