message: Changes API send_tokens to Return Null on Failure
diff: @@ -382,8 +382,7 @@ pub extern fn vcx_wallet_send_tokens(command_handle: u32,
  Err(e) => {
  let msg = "Failed to send tokens".to_string();
  info!("vcx_wallet_send_tokens_cb(command_handle: {}, rc: {}, reciept: {})", command_handle, e.to_error_code(), msg);
  - let msg = CStringUtils::string_to_cstring(msg);
  - cb(command_handle, e.to_error_code(), msg.as_ptr());
  + cb(command_handle, e.to_error_code(), ptr::null());
  },
  }
  });
  