Copter: precland checks get_alt_cm return value

This makes it easier for Covarity to recognise that we don't have an error by checking the return value of get_alt_cm
No functional change

ArduCopter/precision_landing.cpp

@@ -21,7 +21,9 @@ void Copter::update_precland()
     if (rangefinder_alt_ok()) {
         height_above_ground_cm = rangefinder_state.alt_cm;
     } else if (terrain_use()) {
-        current_loc.get_alt_cm(Location_Class::ALT_FRAME_ABOVE_TERRAIN, height_above_ground_cm);
+        if (!current_loc.get_alt_cm(Location_Class::ALT_FRAME_ABOVE_TERRAIN, height_above_ground_cm)) {
+            height_above_ground_cm = current_loc.alt;
+        }
     }
 
     copter.precland.update(height_above_ground_cm);