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/drivers/gps/gps.cpp

@@ -175,9 +175,9 @@ GPS::GPS(const char *uart_path, bool fake_gps, bool enable_sat_info) :
 	_mode(GPS_DRIVER_MODE_UBX),
 	_Helper(nullptr),
 	_Sat_Info(nullptr),
-	_report_gps_pos_pub(-1),
+	_report_gps_pos_pub(0),
 	_p_report_sat_info(nullptr),
-	_report_sat_info_pub(-1),
+	_report_sat_info_pub(0),
 	_rate(0.0f),
 	_fake_gps(fake_gps)
 {