Add missing returns for failure cases in write_some_at and read_some_at.

asio/include/asio/detail/win_iocp_handle_service.hpp

diff --git a/asio/include/asio/detail/win_iocp_handle_service.hpp b/asio/include/asio/detail/win_iocp_handle_service.hpp
index 7f5ad34..1b2febf 100644
--- a/asio/include/asio/detail/win_iocp_handle_service.hpp
+++ b/asio/include/asio/detail/win_iocp_handle_service.hpp
@@ -356,6 +356,7 @@
       DWORD last_error = ::GetLastError();
       ec = asio::error_code(last_error,
           asio::error::get_system_category());
+      return 0;
     }
 
     ec = asio::error_code();
@@ -603,6 +604,7 @@
         ec = asio::error_code(last_error,
             asio::error::get_system_category());
       }
+      return 0;
     }
 
     ec = asio::error_code();