FIX kMaxGearSize and reset callback

common/speedometer.cpp

@@ -25,6 +25,7 @@
 #include "speedometer.hpp"
 
 #include <chrono>
+#include <cstddef>
 #include <ratio>
 
 #include "static_scheduling/gear_device.hpp"
@@ -81,7 +82,9 @@ float Speedometer::getDistance() {
 
 void Speedometer::reset() {
 #if defined(MBED_TEST_MODE)
-    _cbOnReset();
+    if (_cbOnReset != NULL) {
+        _cbOnReset();
+    }
 #endif
 
     this->_totalDistanceMutex.lock();