AC_PrecLand: initialise some members

This was not a real problem but it resolves some Covarity warnings

libraries/AC_PrecLand/AC_PrecLand.cpp

@@ -33,6 +33,7 @@ AC_PrecLand::AC_PrecLand(const AP_AHRS& ahrs, const AP_InertialNav& inav) :
     _ahrs(ahrs),
     _inav(inav),
     _last_update_ms(0),
+    _last_backend_los_meas_ms(0),
     _backend(NULL)
 {
     // set parameters to defaults

libraries/AC_PrecLand/AC_PrecLand_Companion.cpp

@@ -9,7 +9,8 @@ AC_PrecLand_Companion::AC_PrecLand_Companion(const AC_PrecLand& frontend, AC_Pre
     : AC_PrecLand_Backend(frontend, state),
       _timestamp_us(0),
       _distance_to_target(0.0f),
-      _have_los_meas(false)
+      _have_los_meas(false),
+      _los_meas_time_ms(0)
 {
 }