extensive orb_advert_t fixes

The calls to orb_advertise were being mishandled throughout the code.
There were ::close() calls on memory pointers, there were checks
against < 0 when it is a pointer to a object and values larger than
0x7ffffffff are valid. Some places orb_advert_t variables were
being initialized as 0 other places as -1.

The orb_advert_t type was changed to uintptr_t so the pointer value
would not be wrapped as a negative number. This was causing a failure
on ARM.

Tests for < 0 were changed to == 0 since a null pointer is the valid
representation for error, or uninitialized.

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

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -307,9 +307,9 @@ MulticopterPositionControl::MulticopterPositionControl() :
 	_global_vel_sp_sub(-1),
 
 /* publications */
-	_att_sp_pub(-1),
-	_local_pos_sp_pub(-1),
-	_global_vel_sp_pub(-1),
+	_att_sp_pub(0),
+	_local_pos_sp_pub(0),
+	_global_vel_sp_pub(0),
 
 	_ref_alt(0.0f),
 	_ref_timestamp(0),