orb_advert_t changed to void * and checks changed to nullptr

The existing orb_advert_t use thoughout the code sometimes tries
to treat it as a file descriptor and there are checks for < 0
and ::close calls on orb_advert_t types which is an invalid use
of an object pointer, which is what orb_advert_t really is.

Initially I had changed the -1 initializations to 0 but it was
suggested that this should be nullptr. That was a good recommendation
but the definition of orb_advert_t had to change to void * because
you cannot initialize a uintptr_t as nullptr.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/uavcan/actuators/esc.cpp

@@ -181,7 +181,7 @@ void UavcanEscController::orb_pub_timer_cb(const uavcan::TimerEvent&)
 	_esc_status.counter += 1;
 	_esc_status.esc_connectiontype = ESC_CONNECTION_TYPE_CAN;
 
-	if (_esc_status_pub > 0) {
+	if (_esc_status_pub != nullptr) {
 		(void)orb_publish(ORB_ID(esc_status), _esc_status_pub, &_esc_status);
 	} else {
 		_esc_status_pub = orb_advertise(ORB_ID(esc_status), &_esc_status);

src/modules/uavcan/actuators/esc.hpp

@@ -88,7 +88,7 @@ private:
 
 	bool		_armed = false;
 	esc_status_s	_esc_status = {};
-	orb_advert_t	_esc_status_pub = -1;
+	orb_advert_t	_esc_status_pub = nullptr;
 
 	/*
 	 * libuavcan related things