AP_Terrain: prevent valgrind error when terrain not available

libraries/AP_Terrain/TerrainGCS.cpp

@@ -204,11 +204,15 @@ void AP_Terrain::send_terrain_report(mavlink_channel_t chan, const Location &loc
     get_statistics(pending, loaded);
 
     float current_height;
+    if (spacing == 0) {
+        current_height = 0;
+    } else {
         if (current_loc.flags.relative_alt) {
             current_height = current_loc.alt*0.01f;
         } else {
             current_height = (current_loc.alt - ahrs.get_home().alt)*0.01f;
         }
+    }
     current_height += home_terrain_height - terrain_height;
 
     if (comm_get_txspace(chan) >= MAVLINK_NUM_NON_PAYLOAD_BYTES + MAVLINK_MSG_ID_TERRAIN_REPORT_LEN) {