Update sshpty.c

[weedcookie]

According to man pages open return -1 on error, which means a value bigger than or equal to zero is a valid file descriptor.

[daztucker]

According to man pages open return -1 on error

That's true, however your patch is incorrect. If you look at what it's trying to do:

• Verify that we are successfully disconnected from the controlling tty.

/dev/tty is present when a process has controlling tty. This means that if the open succeeds then the process still has a controlling tty, which means that the attempt to disconnect from it failed.