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/sensors/sensor_bridge.hpp

@@ -89,7 +89,7 @@ class UavcanCDevSensorBridgeBase : public IUavcanSensorBridge, public device::CD
 	struct Channel
 	{
 		int node_id              = -1;
-		orb_advert_t orb_advert  = -1;
+		orb_advert_t orb_advert  = nullptr;
 		int class_instance       = -1;
 		int orb_instance	 = -1;
 	};