AP_GPS: ensure constructors zero all key variables

this prevents a crash on PX4 if a GPS is attached after startup

libraries/AP_GPS/AP_GPS_406.h

@@ -21,6 +21,8 @@
 class AP_GPS_406 : public AP_GPS_SIRF
 {
 public:
+    AP_GPS_406() : AP_GPS_SIRF() {}
+
     // Methods
     virtual void        init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting = GPS_ENGINE_NONE);
 

libraries/AP_GPS/AP_GPS_Auto.cpp

@@ -17,6 +17,7 @@ const prog_char AP_GPS_Auto::_sirf_set_binary[]  PROGMEM = SIRF_SET_BINARY;
 
 
 AP_GPS_Auto::AP_GPS_Auto(GPS **gps)  :
+	GPS(),
     _gps(gps)
 {
 }

libraries/AP_GPS/AP_GPS_HIL.h

@@ -17,6 +17,11 @@
 
 class AP_GPS_HIL : public GPS {
 public:
+    AP_GPS_HIL() : 
+		GPS(),
+		_updated(false)
+		{}
+
     virtual void        init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting = GPS_ENGINE_NONE);
     virtual bool        read(void);
 

libraries/AP_GPS/AP_GPS_MTK.h

@@ -20,6 +20,12 @@
 
 class AP_GPS_MTK : public GPS {
 public:
+    AP_GPS_MTK() :
+		GPS(),
+		_step(0),
+		_payload_counter(0)
+		{}
+
     virtual void        init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting = GPS_ENGINE_NONE);
     virtual bool        read(void);
     static bool _detect(uint8_t );

libraries/AP_GPS/AP_GPS_MTK19.cpp

@@ -17,12 +17,6 @@
 #include "AP_GPS_MTK19.h"
 #include <stdint.h>
 
-// Constructors ////////////////////////////////////////////////////////////////
-AP_GPS_MTK19::AP_GPS_MTK19() : GPS()
-{
-	_step = 0;
-}
-
 // Public Methods //////////////////////////////////////////////////////////////
 void
 AP_GPS_MTK19::init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting)

libraries/AP_GPS/AP_GPS_MTK19.h

@@ -22,7 +22,13 @@
 
 class AP_GPS_MTK19 : public GPS {
 public:
-    AP_GPS_MTK19();
+    AP_GPS_MTK19() :
+		GPS(),
+		_step(0),
+		_payload_counter(0),
+		_mtk_revision(0)
+		{}
+
     virtual void        init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting = GPS_ENGINE_NONE);
     virtual bool        read(void);
     static bool 		_detect(uint8_t );

libraries/AP_GPS/AP_GPS_NMEA.h

@@ -52,6 +52,16 @@
 class AP_GPS_NMEA : public GPS
 {
 public:
+	AP_GPS_NMEA(void) : 
+	GPS(),
+	_parity(0),
+	_is_checksum_term(false),
+	_sentence_type(0),
+	_term_number(0),
+	_term_offset(0),
+	_gps_data_good(false)
+		{}
+
     /// Perform a (re)initialisation of the GPS; sends the
     /// protocol configuration messages.
     ///

libraries/AP_GPS/AP_GPS_None.h

@@ -9,6 +9,8 @@
 class AP_GPS_None : public GPS
 {
 public:
+	AP_GPS_None() : GPS() {}
+
     virtual void        init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting = GPS_ENGINE_NONE) {
 		_port = s;
     };

libraries/AP_GPS/AP_GPS_SIRF.h

@@ -18,6 +18,15 @@
 
 class AP_GPS_SIRF : public GPS {
 public:
+	AP_GPS_SIRF() : 
+		GPS(),
+		_step(0),
+		_gather(false),
+		_payload_length(0),
+		_payload_counter(0),
+		_msg_id(0)
+		{}
+
     virtual void        init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting = GPS_ENGINE_NONE);
     virtual bool        read();
 	static bool         _detect(uint8_t data);

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -27,6 +27,17 @@
 class AP_GPS_UBLOX : public GPS
 {
 public:
+	AP_GPS_UBLOX() :
+		GPS(),
+		_step(0),
+		_msg_id(0),
+		_payload_length(0),
+		_payload_counter(0),
+		_fix_count(0),
+		_disable_counter(0),
+		next_fix(false)
+		{}
+
     // Methods
     virtual void                    init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting = GPS_ENGINE_NONE);
     virtual bool                    read();

libraries/AP_GPS/GPS.cpp

@@ -17,6 +17,22 @@ extern const AP_HAL::HAL& hal;
  # define Debug(fmt, args ...)
 #endif
 
+GPS::GPS(void) :
+	// ensure all the inherited fields are zeroed
+	num_sats(0),
+	new_data(false),
+	fix(false),
+	valid_read(false),
+	last_fix_time(0),
+	_have_raw_velocity(false),
+	_status(GPS::NO_FIX),
+	_last_ground_speed_cm(0),
+	_velocity_north(0),
+	_velocity_east(0),
+	_velocity_down(0)	
+{
+}
+
 void
 GPS::update(void)
 {

libraries/AP_GPS/GPS.h

@@ -16,6 +16,7 @@
 class GPS
 {
 public:
+	GPS();
 
     /// Update GPS state based on possible bytes received from the module.
     ///