Cygwin: pty: Fix segfault caused when tcflush() is called.

- After commit 253352e796, mc (midnight
  commander) crashes with segfault if the shell is bash. This is due
  to NULL pointer access in read(). This patch fixes the issue.
  Addresses::
    https://cygwin.com/pipermail/cygwin/2021-February/247870.html

winsup/cygwin/fhandler_tty.cc

@@ -1474,9 +1474,12 @@ wait_retry:
 out:
   termios_printf ("%d = read(%p, %lu)", totalread, ptr, len);
   len = (size_t) totalread;
+  if (ptr0)
+    { /* Not tcflush() */
       bool saw_eol = totalread > 0 && strchr ("\r\n", ptr0[totalread -1]);
       mask_switch_to_pcon_in (false, saw_eol);
     }
+}
 
 int
 fhandler_pty_slave::dup (fhandler_base *child, int flags)