AP_GPS: initialise _step to zero in all drivers

new() does not zero-fill memory

libraries/AP_GPS/AP_GPS_MTK.cpp

@@ -23,6 +23,8 @@ AP_GPS_MTK::init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting)
 {
 	_port = s;
     _port->flush();
+	_step = 0;
+
     // initialize serial port for binary protocol use
     // XXX should assume binary, let GPS_AUTO handle dynamic config?
     _port->print(MTK_SET_BINARY);

libraries/AP_GPS/AP_GPS_MTK16.cpp

@@ -22,6 +22,7 @@ AP_GPS_MTK16::init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting)
 {
 	_port = s;
     _port->flush();
+	_step = 0;
 
     // initialize serial port for binary protocol use
     // XXX should assume binary, let GPS_AUTO handle dynamic config?

libraries/AP_GPS/AP_GPS_MTK19.cpp

@@ -18,6 +18,7 @@
 // Constructors ////////////////////////////////////////////////////////////////
 AP_GPS_MTK19::AP_GPS_MTK19() : GPS()
 {
+	_step = 0;
 }
 
 // Public Methods //////////////////////////////////////////////////////////////

libraries/AP_GPS/AP_GPS_SIRF.cpp

@@ -29,6 +29,7 @@ AP_GPS_SIRF::init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting)
 {
 	_port = s;
     _port->flush();
+	_step = 0;
 
     // For modules that default to something other than SiRF binary,
     // the module-specific subclass should take care of switching to binary mode

libraries/AP_GPS/AP_GPS_UBLOX.cpp

@@ -50,6 +50,7 @@ AP_GPS_UBLOX::init(AP_HAL::UARTDriver *s, enum GPS_Engine_Setting nav_setting)
     _configure_gps();
 
     _nav_setting = nav_setting;
+	_step = 0;
 }
 
 // Process bytes available from the stream