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

[Takashi Yano]

- After commit 253352e796ff9ec9a447e5375f5bc3e2b92b5293, 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 | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/fhandler_tty.cc b/winsup/cygwin/fhandler_tty.cc
index d30041af1..3fcaa8277 100644
--- a/winsup/cygwin/fhandler_tty.cc
+++ b/winsup/cygwin/fhandler_tty.cc
@@ -1474,8 +1474,11 @@ wait_retry:
 out:
   termios_printf ("%d = read(%p, %lu)", totalread, ptr, len);
   len = (size_t) totalread;
-  bool saw_eol = totalread > 0 && strchr ("\r\n", ptr0[totalread -1]);
-  mask_switch_to_pcon_in (false, saw_eol);
+  if (ptr0)
+    { /* Not tcflush() */
+      bool saw_eol = totalread > 0 && strchr ("\r\n", ptr0[totalread -1]);
+      mask_switch_to_pcon_in (false, saw_eol);
+    }
 }
 
 int
-- 
2.30.0