AP_Terrain: avoid direct use of Location alt field

libraries/AP_Terrain/TerrainGCS.cpp

@@ -247,11 +247,9 @@ void AP_Terrain::send_terrain_report(mavlink_channel_t chan, const Location &loc
     if (spacing == 0 && !(extrapolate && have_current_loc_height)) {
         current_height = 0;
     } else {
-        if (current_loc.relative_alt) {
-            current_height = current_loc.alt*0.01f;
-        } else {
-            current_height = (current_loc.alt - ahrs.get_home().alt)*0.01f;
-        }
+        int32_t height_above_home_cm = 0;
+        UNUSED_RESULT(current_loc.get_alt_cm(Location::AltFrame::ABOVE_HOME, height_above_home_cm));
+        current_height = height_above_home_cm * 0.01f;  // cm -> m
     }
     current_height += home_terrain_height - terrain_height;