Cygwin: pty: Revise code waiting for forwarding by master_fwd_thread.

- Though this rarely happens, sometimes the first printing of non-
  cygwin process does not displayed correctly. To fix this issue,
  the code for waiting for forwarding by master_fwd_thread is revised.

winsup/cygwin/tty.cc

@@ -245,6 +245,7 @@ tty::init ()
   num_pcon_attached_slaves = 0;
   term_code_page = 0;
   need_redraw_screen = false;
+  fwd_done = NULL;
 }
 
 HANDLE